[PATCH] libstdc++: constexpr flat_map and flat_multimap

Patrick Palka ppalka@redhat.com
Thu Dec 18 15:14:59 GMT 2025


On Fri, 12 Dec 2025, Yuao Ma wrote:

> Hi all,
> 
> This patch is a follow-up to r16-6026-gbf9dd44a97400e. It makes
> flat_map and flat_multimap constexpr as part of P3372R3.
> Tested on x86_64-linux. OK for trunk?
> 
> Yuao
> 
> From c2a1c42266bdd87c97868735514b621cec3e30bd Mon Sep 17 00:00:00 2001
> From: Yuao Ma <c8ef@outlook.com>
> Date: Fri, 12 Dec 2025 19:26:01 +0800
> Subject: [PATCH] libstdc++: constexpr flat_map and flat_multimap
> 
> This patch makes flat_map and flat_multimap constexpr as part of P3372R3.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/version.def: Add FTM.
> 	* include/bits/version.h: Regenerate.
> 	* include/std/flat_map: Add constexpr.
> 	* testsuite/23_containers/flat_map/1.cc: Add constexpr test.
> 	* testsuite/23_containers/flat_multimap/1.cc: Add constexpr test.

LGTM

> ---
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  10 ++
>  libstdc++-v3/include/std/flat_map             | 157 +++++++++++++++++-
>  .../testsuite/23_containers/flat_map/1.cc     |  43 ++++-
>  .../23_containers/flat_multimap/1.cc          |  42 ++++-
>  5 files changed, 245 insertions(+), 15 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> index 5b95db19ac6..f97276d80f2 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1351,6 +1351,14 @@ ftms = {
>    };
>  };
>  
> +ftms = {
> +  name = constexpr_flat_map;
> +  values = {
> +    v = 202502;
> +    cxxmin = 26;
> +  };
> +};
> +
>  ftms = {
>    name = constexpr_flat_set;
>    values = {
> diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> index eee9890a72b..97b37e9a62b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1496,6 +1496,16 @@
>  #endif /* !defined(__cpp_lib_constexpr_dynamic_alloc) */
>  #undef __glibcxx_want_constexpr_dynamic_alloc
>  
> +#if !defined(__cpp_lib_constexpr_flat_map)
> +# if (__cplusplus >  202302L)
> +#  define __glibcxx_constexpr_flat_map 202502L
> +#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_flat_map)
> +#   define __cpp_lib_constexpr_flat_map 202502L
> +#  endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_flat_map) */
> +#undef __glibcxx_want_constexpr_flat_map
> +
>  #if !defined(__cpp_lib_constexpr_flat_set)
>  # if (__cplusplus >  202302L)
>  #  define __glibcxx_constexpr_flat_set 202502L
> diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
> index 637c3b7fe0d..e270735a87b 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -33,6 +33,7 @@
>  #pragma GCC system_header
>  #endif
>  
> +#define __glibcxx_want_constexpr_flat_map
>  #define __glibcxx_want_flat_map
>  #include <bits/version.h>
>  
> @@ -108,9 +109,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        class value_compare
>        {
>  	[[no_unique_address]] key_compare _M_comp;
> +	_GLIBCXX26_CONSTEXPR
>  	value_compare(key_compare __c) : _M_comp(__c) { }
>  
>        public:
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	operator()(const_reference __x, const_reference __y) const
>  	{ return _M_comp(__x.first, __y.first); }
> @@ -129,10 +132,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	containers* _M_cont;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	_ClearGuard(containers& __cont)
>  	: _M_cont(std::__addressof(__cont))
>  	{ }
>  
> +	_GLIBCXX26_CONSTEXPR
>  	~_ClearGuard()
>  	{
>  	  if (_M_cont)
> @@ -142,24 +147,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    }
>  	}
>  
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_disable()
>  	{ _M_cont = nullptr; }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        _ClearGuard
>        _M_make_clear_guard()
>        { return _ClearGuard{this->_M_cont}; }
>  
>      public:
>        // constructors
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl() : _Flat_map_impl(key_compare()) { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        explicit
>        _Flat_map_impl(const key_compare& __comp)
>        : _M_cont(), _M_comp(__comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
>  		     const key_compare& __comp = key_compare())
> @@ -169,6 +179,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	_M_sort_uniq();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t,
>  		     key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
> @@ -180,12 +191,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
>  	: _M_cont(), _M_comp(__comp)
>  	{ insert(__first, __last); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
> @@ -193,20 +206,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ insert(__s, __first, __last); }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare())
>  	{ }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp)
>  	: _Flat_map_impl(__comp)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
>        : _Flat_map_impl(__il.begin(), __il.end(), __comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t __s,
>  		     initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
> @@ -216,12 +233,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // constructors with allocators
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	explicit
>  	_Flat_map_impl(const _Alloc& __a)
>  	: _Flat_map_impl(key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_compare& __comp, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a)),
> @@ -229,6 +248,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const _Alloc& __a)
> @@ -236,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const key_compare& __comp,
> @@ -249,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -257,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -271,6 +294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const _Derived& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a, __x._M_cont.keys),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a, __x._M_cont.values)),
> @@ -278,6 +302,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_Derived&& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>
>  		  (__a, std::move(__x._M_cont.keys)),
> @@ -288,6 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(std::move(__first), std::move(__last), key_compare(), __a)
> @@ -295,6 +321,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -303,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
> @@ -311,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
> @@ -320,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare(), __a)
> @@ -327,18 +357,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__comp, __a)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__il, key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -346,6 +379,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const _Alloc& __a)
> @@ -353,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const key_compare& __comp,
> @@ -360,6 +395,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	: _Flat_map_impl(__s, __il.begin(), __il.end(), __comp, __a)
>  	{ }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Derived&
>        operator=(initializer_list<value_type> __il)
>        {
> @@ -369,64 +405,79 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        // iterators
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        begin() noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        begin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        end() noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        end() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rbegin() noexcept
>        { return reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rbegin() const noexcept
>        { return const_reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rend() noexcept
>        { return reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rend() const noexcept
>        { return const_reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cbegin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cend() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crbegin() const noexcept
>        { return const_reverse_iterator(cend()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crend() const noexcept
>        { return const_reverse_iterator(cbegin()); }
>  
>        // capacity
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        empty() const noexcept
>        { return _M_cont.keys.empty(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        size() const noexcept
>        { return _M_cont.keys.size(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        max_size() const noexcept
>        { return std::min<size_type>(keys().max_size(), values().max_size()); }
> @@ -436,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // modifiers
>        template<typename _Key2, typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	_M_try_emplace(optional<const_iterator> __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -491,6 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<value_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	emplace(_Args&&... __args)
>  	{
> @@ -503,6 +556,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	emplace_hint(const_iterator __position, _Args&&... __args)
>  	{
> @@ -510,36 +564,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return _M_try_emplace(__position, std::move(__p.first), std::move(__p.second)).first;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(const value_type& __x)
>        { return emplace(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(value_type&& __x)
>        { return emplace(std::move(__x)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, const value_type& __x)
>        { return emplace_hint(__position, __x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, value_type&& __x)
>        { return emplace_hint(__position, std::move(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	insert(_Arg&& __x)
>  	{ return emplace(std::forward<_Arg>(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert(const_iterator __position, _Arg&& __x)
>  	{ return emplace_hint(__position, std::forward<_Arg>(__x)); }
>  
>      private:
>        template<typename _Iter, typename _Sent>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_insert(_Iter __first, _Sent __last)
>  	{
> @@ -577,11 +638,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>      public:
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(_InputIterator __first, _InputIterator __last)
>  	{ _M_insert(std::move(__first), std::move(__last)); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(__sorted_t, _InputIterator __first, _InputIterator __last)
>  	{
> @@ -590,18 +653,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert_range(_Rg&& __rg)
>  	{ _M_insert(ranges::begin(__rg), ranges::end(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(initializer_list<value_type> __il)
>        { insert(__il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(__sorted_t __s, initializer_list<value_type> __il)
>        { insert(__s, __il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        containers
>        extract() &&
>        {
> @@ -609,6 +676,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return {std::move(_M_cont.keys), std::move(_M_cont.values)};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont)
>        {
> @@ -622,10 +690,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // try_emplace and insert_or_assign defined directly in class flat_map only.
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(iterator __position)
>        { return erase(static_cast<const_iterator>(__position)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __position)
>        {
> @@ -637,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        erase(const key_type& __x)
>        { return erase<const key_type&>(__x); }
> @@ -646,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  || (__transparent_comparator<_Compare>
>  	      && !is_convertible_v<_Key2, iterator>
>  	      && !is_convertible_v<_Key2, const_iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	erase(_Key2&& __x)
>  	{
> @@ -655,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return __n;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __first, const_iterator __last)
>        {
> @@ -667,6 +740,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        swap(_Derived& __y) noexcept
>        {
> @@ -675,6 +749,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	ranges::swap(_M_cont.values, __y._M_cont.values);
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        clear() noexcept
>        {
> @@ -684,32 +759,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // observers
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        key_compare
>        key_comp() const
>        { return _M_comp; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        value_compare
>        value_comp() const
>        { return value_compare(_M_comp); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const key_container_type&
>        keys() const noexcept
>        { return _M_cont.keys; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_container_type&
>        values() const noexcept
>        { return _M_cont.values; }
>  
>        // map operations
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        find(const key_type& __x)
>        { return find<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        find(const key_type& __x) const
>        { return find<key_type>(__x); }
> @@ -717,6 +798,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	find(const _Key2& __x)
>  	{
> @@ -730,6 +812,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	find(const _Key2& __x) const
>  	{
> @@ -741,6 +824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        count(const key_type& __x) const
>        { return count<key_type>(__x); }
> @@ -748,6 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	count(const _Key2& __x) const
>  	{
> @@ -761,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        contains(const key_type& __x) const
>        { return contains<key_type>(__x); }
> @@ -768,16 +854,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	contains(const _Key2& __x) const
>  	{ return find(__x) != cend(); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        lower_bound(const key_type& __x)
>        { return lower_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        lower_bound(const key_type& __x) const
>        { return lower_bound<key_type>(__x); }
> @@ -785,6 +874,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	lower_bound(const _Key2& __x)
>  	{
> @@ -796,6 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	lower_bound(const _Key2& __x) const
>  	{
> @@ -805,11 +896,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        upper_bound(const key_type& __x)
>        { return upper_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        upper_bound(const key_type& __x) const
>        { return upper_bound<key_type>(__x); }
> @@ -817,6 +910,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	upper_bound(const _Key2& __x)
>  	{
> @@ -828,6 +922,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	upper_bound(const _Key2& __x) const
>  	{
> @@ -837,11 +932,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<iterator, iterator>
>        equal_range(const key_type& __x)
>        { return equal_range<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<const_iterator, const_iterator>
>        equal_range(const key_type& __x) const
>        { return equal_range<key_type>(__x); }
> @@ -849,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, iterator>
>  	equal_range(const _Key2& __x)
>  	{
> @@ -861,6 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<const_iterator, const_iterator>
>  	equal_range(const _Key2& __x) const
>  	{
> @@ -871,7 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Derived& __x, const _Derived& __y)
>        {
>  	return __x._M_cont.keys == __y._M_cont.keys
> @@ -880,7 +979,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Up = value_type>
>  	[[nodiscard]]
> -	friend __detail::__synth3way_t<_Up>
> +	friend _GLIBCXX26_CONSTEXPR __detail::__synth3way_t<_Up>
>  	operator<=>(const _Derived& __x, const _Derived& __y)
>  	{
>  	  return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
> @@ -888,11 +987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  							__detail::__synth3way);
>  	}
>  
> -      friend void
> +      friend _GLIBCXX26_CONSTEXPR void
>        swap(_Derived& __x, _Derived& __y) noexcept
>        { return __x.swap(__y); }
>  
>        template<typename _Predicate>
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	_M_erase_if(_Predicate __pred)
>  	{
> @@ -912,6 +1012,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        containers _M_cont;
>        [[no_unique_address]] _Compare _M_comp;
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_sort_uniq()
>        {
> @@ -921,13 +1022,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_unique();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_unique() requires (!_Multi)
>        {
>  	struct __key_equiv
>  	{
> +	  _GLIBCXX26_CONSTEXPR
>  	  __key_equiv(key_compare __c) : _M_comp(__c) { }
>  
> +	  _GLIBCXX26_CONSTEXPR
>  	  bool
>  	  operator()(const_reference __x, const_reference __y) const
>  	  { return !_M_comp(__x.first, __y.first) && !_M_comp(__y.first, __x.first); }
> @@ -958,12 +1062,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  				     ranges::__maybe_const_t<_Const, mapped_type>&>;
>        using difference_type   = ptrdiff_t;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator() = default;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Iterator<!_Const> __it) requires _Const
>        : _M_cont(__it._M_cont), _M_index(__it._M_index)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator*() const noexcept
>        {
> @@ -975,19 +1082,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	reference __p;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	const reference*
>  	operator->() const noexcept
>  	{ return std::__addressof(__p); }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        pointer
>        operator->() const
>        { return pointer{operator*()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator[](difference_type __n) const noexcept
>        { return *(*this + __n); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator++() noexcept
>        {
> @@ -995,6 +1106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator--() noexcept
>        {
> @@ -1002,6 +1114,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator++(int) noexcept
>        {
> @@ -1010,6 +1123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator--(int) noexcept
>        {
> @@ -1018,6 +1132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator+=(difference_type __n) noexcept
>        {
> @@ -1025,6 +1140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator-=(difference_type __n) noexcept
>        {
> @@ -1036,45 +1152,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend _Flat_map_impl;
>        friend _Iterator<!_Const>;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires (!_Const)
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(const _Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires _Const
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(difference_type __n, _Iterator __it) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator-(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it -= __n;
>  	return __it;
>        }
>  
> -      friend difference_type
> +      friend _GLIBCXX26_CONSTEXPR difference_type
>        operator-(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
>  	return __x._M_index - __y._M_index;
>        }
>  
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1082,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __x._M_index == __y._M_index;
>        }
>  
> -      friend strong_ordering
> +      friend _GLIBCXX26_CONSTEXPR strong_ordering
>        operator<=>(const _Iterator& __x, const _Iterator& __y)
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1146,30 +1264,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using _Impl::max_size;
>  
>        // element access
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](const key_type& __x)
>        { return try_emplace(__x).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](key_type&& __x)
>        { return try_emplace(std::move(__x)).first->second; }
>  
>        template<typename _Key2>
>  	requires __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	operator[](_Key2&& __x)
>  	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        at(const key_type& __x)
>        { return at<key_type>(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_type&
>        at(const key_type& __x) const
>        { return at<key_type>(__x); }
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	at(const _Key2& __x)
>  	{
> @@ -1181,6 +1305,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	const mapped_type&
>  	at(const _Key2& __x) const
>  	{
> @@ -1203,12 +1328,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(nullopt, __k, std::forward<_Args>(__args)...); }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(key_type&& __k, _Args&&... __args)
>  	{
> @@ -1222,6 +1349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<mapped_type, _Args...>
>  	  && (!is_convertible_v<_Key2&&, const_iterator>)
>  	  && (!is_convertible_v<_Key2&&, iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(_Key2&& __k, _Args&&... __args)
>  	{
> @@ -1231,12 +1359,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(__hint, __k, std::forward<_Args>(__args)...).first; }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
>  	{
> @@ -1248,6 +1378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	requires __transparent_comparator<_Compare>
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -1258,6 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(const key_type& __k, _Mapped&& __obj)
>  	{ return insert_or_assign<const key_type&, _Mapped>(__k, std::forward<_Mapped>(__obj)); }
> @@ -1265,6 +1397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1277,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(_Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1290,6 +1424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, const key_type& __k, _Mapped&& __obj)
>  	{
> @@ -1300,6 +1435,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1312,6 +1448,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, _Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1422,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> @@ -1590,6 +1728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> index 5bcda3464f1..04c2101aab1 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> @@ -17,7 +17,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -76,6 +76,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -127,6 +128,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(20,42) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -158,6 +160,7 @@ test02()
>    VERIFY( m.count(3) == 1 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -186,6 +189,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -218,6 +222,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -227,6 +232,7 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, -4, -5}) );
>  }
>  
> +constexpr
>  void
>  test06()
>  {
> @@ -243,6 +249,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -254,6 +261,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
>  }
>  
> +constexpr
>  void
>  test08()
>  {
> @@ -263,6 +271,7 @@ test08()
>    m[k] = 0;
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -274,8 +283,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -290,3 +299,31 @@ main()
>    test08();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test08();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> index f143f3acd1b..00644c14061 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> @@ -11,7 +11,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -74,6 +74,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -111,6 +112,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(12,3) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -137,6 +139,7 @@ test02()
>    VERIFY( m.count(3) == 2 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -165,6 +168,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -197,6 +201,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -205,6 +210,8 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::keys, (int[]){1, 2, 3, 3, 4, 5}) );
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, 3, -4, -5}) );
>  }
> +
> +constexpr
>  void
>  test06()
>  {
> @@ -221,6 +228,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -232,6 +240,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -243,8 +252,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -258,3 +267,30 @@ main()
>    test07();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> -- 
> 2.52.0
> 
> From c2a1c42266bdd87c97868735514b621cec3e30bd Mon Sep 17 00:00:00 2001
> From: Yuao Ma <c8ef@outlook.com>
> Date: Fri, 12 Dec 2025 19:26:01 +0800
> Subject: [PATCH] libstdc++: constexpr flat_map and flat_multimap
> 
> This patch makes flat_map and flat_multimap constexpr as part of P3372R3.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/version.def: Add FTM.
> 	* include/bits/version.h: Regenerate.
> 	* include/std/flat_map: Add constexpr.
> 	* testsuite/23_containers/flat_map/1.cc: Add constexpr test.
> 	* testsuite/23_containers/flat_multimap/1.cc: Add constexpr test.
> ---
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  10 ++
>  libstdc++-v3/include/std/flat_map             | 157 +++++++++++++++++-
>  .../testsuite/23_containers/flat_map/1.cc     |  43 ++++-
>  .../23_containers/flat_multimap/1.cc          |  42 ++++-
>  5 files changed, 245 insertions(+), 15 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> index 5b95db19ac6..f97276d80f2 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1351,6 +1351,14 @@ ftms = {
>    };
>  };
>  
> +ftms = {
> +  name = constexpr_flat_map;
> +  values = {
> +    v = 202502;
> +    cxxmin = 26;
> +  };
> +};
> +
>  ftms = {
>    name = constexpr_flat_set;
>    values = {
> diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> index eee9890a72b..97b37e9a62b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1496,6 +1496,16 @@
>  #endif /* !defined(__cpp_lib_constexpr_dynamic_alloc) */
>  #undef __glibcxx_want_constexpr_dynamic_alloc
>  
> +#if !defined(__cpp_lib_constexpr_flat_map)
> +# if (__cplusplus >  202302L)
> +#  define __glibcxx_constexpr_flat_map 202502L
> +#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_flat_map)
> +#   define __cpp_lib_constexpr_flat_map 202502L
> +#  endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_flat_map) */
> +#undef __glibcxx_want_constexpr_flat_map
> +
>  #if !defined(__cpp_lib_constexpr_flat_set)
>  # if (__cplusplus >  202302L)
>  #  define __glibcxx_constexpr_flat_set 202502L
> diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
> index 637c3b7fe0d..e270735a87b 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -33,6 +33,7 @@
>  #pragma GCC system_header
>  #endif
>  
> +#define __glibcxx_want_constexpr_flat_map
>  #define __glibcxx_want_flat_map
>  #include <bits/version.h>
>  
> @@ -108,9 +109,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        class value_compare
>        {
>  	[[no_unique_address]] key_compare _M_comp;
> +	_GLIBCXX26_CONSTEXPR
>  	value_compare(key_compare __c) : _M_comp(__c) { }
>  
>        public:
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	operator()(const_reference __x, const_reference __y) const
>  	{ return _M_comp(__x.first, __y.first); }
> @@ -129,10 +132,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	containers* _M_cont;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	_ClearGuard(containers& __cont)
>  	: _M_cont(std::__addressof(__cont))
>  	{ }
>  
> +	_GLIBCXX26_CONSTEXPR
>  	~_ClearGuard()
>  	{
>  	  if (_M_cont)
> @@ -142,24 +147,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    }
>  	}
>  
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_disable()
>  	{ _M_cont = nullptr; }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        _ClearGuard
>        _M_make_clear_guard()
>        { return _ClearGuard{this->_M_cont}; }
>  
>      public:
>        // constructors
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl() : _Flat_map_impl(key_compare()) { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        explicit
>        _Flat_map_impl(const key_compare& __comp)
>        : _M_cont(), _M_comp(__comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
>  		     const key_compare& __comp = key_compare())
> @@ -169,6 +179,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	_M_sort_uniq();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t,
>  		     key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
> @@ -180,12 +191,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
>  	: _M_cont(), _M_comp(__comp)
>  	{ insert(__first, __last); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
> @@ -193,20 +206,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ insert(__s, __first, __last); }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare())
>  	{ }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp)
>  	: _Flat_map_impl(__comp)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
>        : _Flat_map_impl(__il.begin(), __il.end(), __comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t __s,
>  		     initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
> @@ -216,12 +233,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // constructors with allocators
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	explicit
>  	_Flat_map_impl(const _Alloc& __a)
>  	: _Flat_map_impl(key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_compare& __comp, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a)),
> @@ -229,6 +248,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const _Alloc& __a)
> @@ -236,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const key_compare& __comp,
> @@ -249,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -257,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -271,6 +294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const _Derived& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a, __x._M_cont.keys),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a, __x._M_cont.values)),
> @@ -278,6 +302,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_Derived&& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>
>  		  (__a, std::move(__x._M_cont.keys)),
> @@ -288,6 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(std::move(__first), std::move(__last), key_compare(), __a)
> @@ -295,6 +321,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -303,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
> @@ -311,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
> @@ -320,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare(), __a)
> @@ -327,18 +357,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__comp, __a)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__il, key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -346,6 +379,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const _Alloc& __a)
> @@ -353,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const key_compare& __comp,
> @@ -360,6 +395,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	: _Flat_map_impl(__s, __il.begin(), __il.end(), __comp, __a)
>  	{ }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Derived&
>        operator=(initializer_list<value_type> __il)
>        {
> @@ -369,64 +405,79 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        // iterators
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        begin() noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        begin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        end() noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        end() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rbegin() noexcept
>        { return reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rbegin() const noexcept
>        { return const_reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rend() noexcept
>        { return reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rend() const noexcept
>        { return const_reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cbegin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cend() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crbegin() const noexcept
>        { return const_reverse_iterator(cend()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crend() const noexcept
>        { return const_reverse_iterator(cbegin()); }
>  
>        // capacity
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        empty() const noexcept
>        { return _M_cont.keys.empty(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        size() const noexcept
>        { return _M_cont.keys.size(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        max_size() const noexcept
>        { return std::min<size_type>(keys().max_size(), values().max_size()); }
> @@ -436,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // modifiers
>        template<typename _Key2, typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	_M_try_emplace(optional<const_iterator> __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -491,6 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<value_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	emplace(_Args&&... __args)
>  	{
> @@ -503,6 +556,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	emplace_hint(const_iterator __position, _Args&&... __args)
>  	{
> @@ -510,36 +564,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return _M_try_emplace(__position, std::move(__p.first), std::move(__p.second)).first;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(const value_type& __x)
>        { return emplace(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(value_type&& __x)
>        { return emplace(std::move(__x)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, const value_type& __x)
>        { return emplace_hint(__position, __x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, value_type&& __x)
>        { return emplace_hint(__position, std::move(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	insert(_Arg&& __x)
>  	{ return emplace(std::forward<_Arg>(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert(const_iterator __position, _Arg&& __x)
>  	{ return emplace_hint(__position, std::forward<_Arg>(__x)); }
>  
>      private:
>        template<typename _Iter, typename _Sent>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_insert(_Iter __first, _Sent __last)
>  	{
> @@ -577,11 +638,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>      public:
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(_InputIterator __first, _InputIterator __last)
>  	{ _M_insert(std::move(__first), std::move(__last)); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(__sorted_t, _InputIterator __first, _InputIterator __last)
>  	{
> @@ -590,18 +653,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert_range(_Rg&& __rg)
>  	{ _M_insert(ranges::begin(__rg), ranges::end(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(initializer_list<value_type> __il)
>        { insert(__il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(__sorted_t __s, initializer_list<value_type> __il)
>        { insert(__s, __il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        containers
>        extract() &&
>        {
> @@ -609,6 +676,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return {std::move(_M_cont.keys), std::move(_M_cont.values)};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont)
>        {
> @@ -622,10 +690,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // try_emplace and insert_or_assign defined directly in class flat_map only.
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(iterator __position)
>        { return erase(static_cast<const_iterator>(__position)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __position)
>        {
> @@ -637,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        erase(const key_type& __x)
>        { return erase<const key_type&>(__x); }
> @@ -646,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  || (__transparent_comparator<_Compare>
>  	      && !is_convertible_v<_Key2, iterator>
>  	      && !is_convertible_v<_Key2, const_iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	erase(_Key2&& __x)
>  	{
> @@ -655,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return __n;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __first, const_iterator __last)
>        {
> @@ -667,6 +740,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        swap(_Derived& __y) noexcept
>        {
> @@ -675,6 +749,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	ranges::swap(_M_cont.values, __y._M_cont.values);
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        clear() noexcept
>        {
> @@ -684,32 +759,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // observers
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        key_compare
>        key_comp() const
>        { return _M_comp; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        value_compare
>        value_comp() const
>        { return value_compare(_M_comp); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const key_container_type&
>        keys() const noexcept
>        { return _M_cont.keys; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_container_type&
>        values() const noexcept
>        { return _M_cont.values; }
>  
>        // map operations
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        find(const key_type& __x)
>        { return find<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        find(const key_type& __x) const
>        { return find<key_type>(__x); }
> @@ -717,6 +798,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	find(const _Key2& __x)
>  	{
> @@ -730,6 +812,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	find(const _Key2& __x) const
>  	{
> @@ -741,6 +824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        count(const key_type& __x) const
>        { return count<key_type>(__x); }
> @@ -748,6 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	count(const _Key2& __x) const
>  	{
> @@ -761,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        contains(const key_type& __x) const
>        { return contains<key_type>(__x); }
> @@ -768,16 +854,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	contains(const _Key2& __x) const
>  	{ return find(__x) != cend(); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        lower_bound(const key_type& __x)
>        { return lower_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        lower_bound(const key_type& __x) const
>        { return lower_bound<key_type>(__x); }
> @@ -785,6 +874,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	lower_bound(const _Key2& __x)
>  	{
> @@ -796,6 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	lower_bound(const _Key2& __x) const
>  	{
> @@ -805,11 +896,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        upper_bound(const key_type& __x)
>        { return upper_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        upper_bound(const key_type& __x) const
>        { return upper_bound<key_type>(__x); }
> @@ -817,6 +910,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	upper_bound(const _Key2& __x)
>  	{
> @@ -828,6 +922,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	upper_bound(const _Key2& __x) const
>  	{
> @@ -837,11 +932,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<iterator, iterator>
>        equal_range(const key_type& __x)
>        { return equal_range<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<const_iterator, const_iterator>
>        equal_range(const key_type& __x) const
>        { return equal_range<key_type>(__x); }
> @@ -849,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, iterator>
>  	equal_range(const _Key2& __x)
>  	{
> @@ -861,6 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<const_iterator, const_iterator>
>  	equal_range(const _Key2& __x) const
>  	{
> @@ -871,7 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Derived& __x, const _Derived& __y)
>        {
>  	return __x._M_cont.keys == __y._M_cont.keys
> @@ -880,7 +979,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Up = value_type>
>  	[[nodiscard]]
> -	friend __detail::__synth3way_t<_Up>
> +	friend _GLIBCXX26_CONSTEXPR __detail::__synth3way_t<_Up>
>  	operator<=>(const _Derived& __x, const _Derived& __y)
>  	{
>  	  return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
> @@ -888,11 +987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  							__detail::__synth3way);
>  	}
>  
> -      friend void
> +      friend _GLIBCXX26_CONSTEXPR void
>        swap(_Derived& __x, _Derived& __y) noexcept
>        { return __x.swap(__y); }
>  
>        template<typename _Predicate>
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	_M_erase_if(_Predicate __pred)
>  	{
> @@ -912,6 +1012,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        containers _M_cont;
>        [[no_unique_address]] _Compare _M_comp;
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_sort_uniq()
>        {
> @@ -921,13 +1022,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_unique();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_unique() requires (!_Multi)
>        {
>  	struct __key_equiv
>  	{
> +	  _GLIBCXX26_CONSTEXPR
>  	  __key_equiv(key_compare __c) : _M_comp(__c) { }
>  
> +	  _GLIBCXX26_CONSTEXPR
>  	  bool
>  	  operator()(const_reference __x, const_reference __y) const
>  	  { return !_M_comp(__x.first, __y.first) && !_M_comp(__y.first, __x.first); }
> @@ -958,12 +1062,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  				     ranges::__maybe_const_t<_Const, mapped_type>&>;
>        using difference_type   = ptrdiff_t;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator() = default;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Iterator<!_Const> __it) requires _Const
>        : _M_cont(__it._M_cont), _M_index(__it._M_index)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator*() const noexcept
>        {
> @@ -975,19 +1082,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	reference __p;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	const reference*
>  	operator->() const noexcept
>  	{ return std::__addressof(__p); }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        pointer
>        operator->() const
>        { return pointer{operator*()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator[](difference_type __n) const noexcept
>        { return *(*this + __n); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator++() noexcept
>        {
> @@ -995,6 +1106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator--() noexcept
>        {
> @@ -1002,6 +1114,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator++(int) noexcept
>        {
> @@ -1010,6 +1123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator--(int) noexcept
>        {
> @@ -1018,6 +1132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator+=(difference_type __n) noexcept
>        {
> @@ -1025,6 +1140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator-=(difference_type __n) noexcept
>        {
> @@ -1036,45 +1152,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend _Flat_map_impl;
>        friend _Iterator<!_Const>;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires (!_Const)
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(const _Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires _Const
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(difference_type __n, _Iterator __it) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator-(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it -= __n;
>  	return __it;
>        }
>  
> -      friend difference_type
> +      friend _GLIBCXX26_CONSTEXPR difference_type
>        operator-(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
>  	return __x._M_index - __y._M_index;
>        }
>  
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1082,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __x._M_index == __y._M_index;
>        }
>  
> -      friend strong_ordering
> +      friend _GLIBCXX26_CONSTEXPR strong_ordering
>        operator<=>(const _Iterator& __x, const _Iterator& __y)
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1146,30 +1264,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using _Impl::max_size;
>  
>        // element access
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](const key_type& __x)
>        { return try_emplace(__x).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](key_type&& __x)
>        { return try_emplace(std::move(__x)).first->second; }
>  
>        template<typename _Key2>
>  	requires __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	operator[](_Key2&& __x)
>  	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        at(const key_type& __x)
>        { return at<key_type>(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_type&
>        at(const key_type& __x) const
>        { return at<key_type>(__x); }
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	at(const _Key2& __x)
>  	{
> @@ -1181,6 +1305,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	const mapped_type&
>  	at(const _Key2& __x) const
>  	{
> @@ -1203,12 +1328,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(nullopt, __k, std::forward<_Args>(__args)...); }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(key_type&& __k, _Args&&... __args)
>  	{
> @@ -1222,6 +1349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<mapped_type, _Args...>
>  	  && (!is_convertible_v<_Key2&&, const_iterator>)
>  	  && (!is_convertible_v<_Key2&&, iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(_Key2&& __k, _Args&&... __args)
>  	{
> @@ -1231,12 +1359,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(__hint, __k, std::forward<_Args>(__args)...).first; }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
>  	{
> @@ -1248,6 +1378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	requires __transparent_comparator<_Compare>
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -1258,6 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(const key_type& __k, _Mapped&& __obj)
>  	{ return insert_or_assign<const key_type&, _Mapped>(__k, std::forward<_Mapped>(__obj)); }
> @@ -1265,6 +1397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1277,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(_Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1290,6 +1424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, const key_type& __k, _Mapped&& __obj)
>  	{
> @@ -1300,6 +1435,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1312,6 +1448,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, _Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1422,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> @@ -1590,6 +1728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> index 5bcda3464f1..04c2101aab1 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> @@ -17,7 +17,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -76,6 +76,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -127,6 +128,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(20,42) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -158,6 +160,7 @@ test02()
>    VERIFY( m.count(3) == 1 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -186,6 +189,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -218,6 +222,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -227,6 +232,7 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, -4, -5}) );
>  }
>  
> +constexpr
>  void
>  test06()
>  {
> @@ -243,6 +249,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -254,6 +261,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
>  }
>  
> +constexpr
>  void
>  test08()
>  {
> @@ -263,6 +271,7 @@ test08()
>    m[k] = 0;
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -274,8 +283,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -290,3 +299,31 @@ main()
>    test08();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test08();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> index f143f3acd1b..00644c14061 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> @@ -11,7 +11,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -74,6 +74,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -111,6 +112,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(12,3) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -137,6 +139,7 @@ test02()
>    VERIFY( m.count(3) == 2 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -165,6 +168,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -197,6 +201,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -205,6 +210,8 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::keys, (int[]){1, 2, 3, 3, 4, 5}) );
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, 3, -4, -5}) );
>  }
> +
> +constexpr
>  void
>  test06()
>  {
> @@ -221,6 +228,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -232,6 +240,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -243,8 +252,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -258,3 +267,30 @@ main()
>    test07();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> -- 
> 2.52.0
> 
> From c2a1c42266bdd87c97868735514b621cec3e30bd Mon Sep 17 00:00:00 2001
> From: Yuao Ma <c8ef@outlook.com>
> Date: Fri, 12 Dec 2025 19:26:01 +0800
> Subject: [PATCH] libstdc++: constexpr flat_map and flat_multimap
> 
> This patch makes flat_map and flat_multimap constexpr as part of P3372R3.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/version.def: Add FTM.
> 	* include/bits/version.h: Regenerate.
> 	* include/std/flat_map: Add constexpr.
> 	* testsuite/23_containers/flat_map/1.cc: Add constexpr test.
> 	* testsuite/23_containers/flat_multimap/1.cc: Add constexpr test.
> ---
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  10 ++
>  libstdc++-v3/include/std/flat_map             | 157 +++++++++++++++++-
>  .../testsuite/23_containers/flat_map/1.cc     |  43 ++++-
>  .../23_containers/flat_multimap/1.cc          |  42 ++++-
>  5 files changed, 245 insertions(+), 15 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> index 5b95db19ac6..f97276d80f2 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1351,6 +1351,14 @@ ftms = {
>    };
>  };
>  
> +ftms = {
> +  name = constexpr_flat_map;
> +  values = {
> +    v = 202502;
> +    cxxmin = 26;
> +  };
> +};
> +
>  ftms = {
>    name = constexpr_flat_set;
>    values = {
> diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> index eee9890a72b..97b37e9a62b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1496,6 +1496,16 @@
>  #endif /* !defined(__cpp_lib_constexpr_dynamic_alloc) */
>  #undef __glibcxx_want_constexpr_dynamic_alloc
>  
> +#if !defined(__cpp_lib_constexpr_flat_map)
> +# if (__cplusplus >  202302L)
> +#  define __glibcxx_constexpr_flat_map 202502L
> +#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_flat_map)
> +#   define __cpp_lib_constexpr_flat_map 202502L
> +#  endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_flat_map) */
> +#undef __glibcxx_want_constexpr_flat_map
> +
>  #if !defined(__cpp_lib_constexpr_flat_set)
>  # if (__cplusplus >  202302L)
>  #  define __glibcxx_constexpr_flat_set 202502L
> diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
> index 637c3b7fe0d..e270735a87b 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -33,6 +33,7 @@
>  #pragma GCC system_header
>  #endif
>  
> +#define __glibcxx_want_constexpr_flat_map
>  #define __glibcxx_want_flat_map
>  #include <bits/version.h>
>  
> @@ -108,9 +109,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        class value_compare
>        {
>  	[[no_unique_address]] key_compare _M_comp;
> +	_GLIBCXX26_CONSTEXPR
>  	value_compare(key_compare __c) : _M_comp(__c) { }
>  
>        public:
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	operator()(const_reference __x, const_reference __y) const
>  	{ return _M_comp(__x.first, __y.first); }
> @@ -129,10 +132,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	containers* _M_cont;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	_ClearGuard(containers& __cont)
>  	: _M_cont(std::__addressof(__cont))
>  	{ }
>  
> +	_GLIBCXX26_CONSTEXPR
>  	~_ClearGuard()
>  	{
>  	  if (_M_cont)
> @@ -142,24 +147,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    }
>  	}
>  
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_disable()
>  	{ _M_cont = nullptr; }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        _ClearGuard
>        _M_make_clear_guard()
>        { return _ClearGuard{this->_M_cont}; }
>  
>      public:
>        // constructors
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl() : _Flat_map_impl(key_compare()) { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        explicit
>        _Flat_map_impl(const key_compare& __comp)
>        : _M_cont(), _M_comp(__comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
>  		     const key_compare& __comp = key_compare())
> @@ -169,6 +179,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	_M_sort_uniq();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t,
>  		     key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
> @@ -180,12 +191,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
>  	: _M_cont(), _M_comp(__comp)
>  	{ insert(__first, __last); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
> @@ -193,20 +206,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ insert(__s, __first, __last); }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare())
>  	{ }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp)
>  	: _Flat_map_impl(__comp)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
>        : _Flat_map_impl(__il.begin(), __il.end(), __comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t __s,
>  		     initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
> @@ -216,12 +233,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // constructors with allocators
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	explicit
>  	_Flat_map_impl(const _Alloc& __a)
>  	: _Flat_map_impl(key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_compare& __comp, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a)),
> @@ -229,6 +248,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const _Alloc& __a)
> @@ -236,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const key_compare& __comp,
> @@ -249,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -257,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -271,6 +294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const _Derived& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a, __x._M_cont.keys),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a, __x._M_cont.values)),
> @@ -278,6 +302,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_Derived&& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>
>  		  (__a, std::move(__x._M_cont.keys)),
> @@ -288,6 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(std::move(__first), std::move(__last), key_compare(), __a)
> @@ -295,6 +321,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -303,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
> @@ -311,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
> @@ -320,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare(), __a)
> @@ -327,18 +357,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__comp, __a)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__il, key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -346,6 +379,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const _Alloc& __a)
> @@ -353,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const key_compare& __comp,
> @@ -360,6 +395,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	: _Flat_map_impl(__s, __il.begin(), __il.end(), __comp, __a)
>  	{ }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Derived&
>        operator=(initializer_list<value_type> __il)
>        {
> @@ -369,64 +405,79 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        // iterators
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        begin() noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        begin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        end() noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        end() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rbegin() noexcept
>        { return reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rbegin() const noexcept
>        { return const_reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rend() noexcept
>        { return reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rend() const noexcept
>        { return const_reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cbegin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cend() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crbegin() const noexcept
>        { return const_reverse_iterator(cend()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crend() const noexcept
>        { return const_reverse_iterator(cbegin()); }
>  
>        // capacity
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        empty() const noexcept
>        { return _M_cont.keys.empty(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        size() const noexcept
>        { return _M_cont.keys.size(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        max_size() const noexcept
>        { return std::min<size_type>(keys().max_size(), values().max_size()); }
> @@ -436,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // modifiers
>        template<typename _Key2, typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	_M_try_emplace(optional<const_iterator> __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -491,6 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<value_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	emplace(_Args&&... __args)
>  	{
> @@ -503,6 +556,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	emplace_hint(const_iterator __position, _Args&&... __args)
>  	{
> @@ -510,36 +564,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return _M_try_emplace(__position, std::move(__p.first), std::move(__p.second)).first;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(const value_type& __x)
>        { return emplace(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(value_type&& __x)
>        { return emplace(std::move(__x)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, const value_type& __x)
>        { return emplace_hint(__position, __x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, value_type&& __x)
>        { return emplace_hint(__position, std::move(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	insert(_Arg&& __x)
>  	{ return emplace(std::forward<_Arg>(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert(const_iterator __position, _Arg&& __x)
>  	{ return emplace_hint(__position, std::forward<_Arg>(__x)); }
>  
>      private:
>        template<typename _Iter, typename _Sent>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_insert(_Iter __first, _Sent __last)
>  	{
> @@ -577,11 +638,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>      public:
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(_InputIterator __first, _InputIterator __last)
>  	{ _M_insert(std::move(__first), std::move(__last)); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(__sorted_t, _InputIterator __first, _InputIterator __last)
>  	{
> @@ -590,18 +653,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert_range(_Rg&& __rg)
>  	{ _M_insert(ranges::begin(__rg), ranges::end(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(initializer_list<value_type> __il)
>        { insert(__il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(__sorted_t __s, initializer_list<value_type> __il)
>        { insert(__s, __il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        containers
>        extract() &&
>        {
> @@ -609,6 +676,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return {std::move(_M_cont.keys), std::move(_M_cont.values)};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont)
>        {
> @@ -622,10 +690,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // try_emplace and insert_or_assign defined directly in class flat_map only.
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(iterator __position)
>        { return erase(static_cast<const_iterator>(__position)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __position)
>        {
> @@ -637,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        erase(const key_type& __x)
>        { return erase<const key_type&>(__x); }
> @@ -646,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  || (__transparent_comparator<_Compare>
>  	      && !is_convertible_v<_Key2, iterator>
>  	      && !is_convertible_v<_Key2, const_iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	erase(_Key2&& __x)
>  	{
> @@ -655,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return __n;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __first, const_iterator __last)
>        {
> @@ -667,6 +740,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        swap(_Derived& __y) noexcept
>        {
> @@ -675,6 +749,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	ranges::swap(_M_cont.values, __y._M_cont.values);
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        clear() noexcept
>        {
> @@ -684,32 +759,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // observers
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        key_compare
>        key_comp() const
>        { return _M_comp; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        value_compare
>        value_comp() const
>        { return value_compare(_M_comp); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const key_container_type&
>        keys() const noexcept
>        { return _M_cont.keys; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_container_type&
>        values() const noexcept
>        { return _M_cont.values; }
>  
>        // map operations
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        find(const key_type& __x)
>        { return find<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        find(const key_type& __x) const
>        { return find<key_type>(__x); }
> @@ -717,6 +798,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	find(const _Key2& __x)
>  	{
> @@ -730,6 +812,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	find(const _Key2& __x) const
>  	{
> @@ -741,6 +824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        count(const key_type& __x) const
>        { return count<key_type>(__x); }
> @@ -748,6 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	count(const _Key2& __x) const
>  	{
> @@ -761,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        contains(const key_type& __x) const
>        { return contains<key_type>(__x); }
> @@ -768,16 +854,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	contains(const _Key2& __x) const
>  	{ return find(__x) != cend(); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        lower_bound(const key_type& __x)
>        { return lower_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        lower_bound(const key_type& __x) const
>        { return lower_bound<key_type>(__x); }
> @@ -785,6 +874,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	lower_bound(const _Key2& __x)
>  	{
> @@ -796,6 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	lower_bound(const _Key2& __x) const
>  	{
> @@ -805,11 +896,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        upper_bound(const key_type& __x)
>        { return upper_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        upper_bound(const key_type& __x) const
>        { return upper_bound<key_type>(__x); }
> @@ -817,6 +910,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	upper_bound(const _Key2& __x)
>  	{
> @@ -828,6 +922,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	upper_bound(const _Key2& __x) const
>  	{
> @@ -837,11 +932,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<iterator, iterator>
>        equal_range(const key_type& __x)
>        { return equal_range<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<const_iterator, const_iterator>
>        equal_range(const key_type& __x) const
>        { return equal_range<key_type>(__x); }
> @@ -849,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, iterator>
>  	equal_range(const _Key2& __x)
>  	{
> @@ -861,6 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<const_iterator, const_iterator>
>  	equal_range(const _Key2& __x) const
>  	{
> @@ -871,7 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Derived& __x, const _Derived& __y)
>        {
>  	return __x._M_cont.keys == __y._M_cont.keys
> @@ -880,7 +979,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Up = value_type>
>  	[[nodiscard]]
> -	friend __detail::__synth3way_t<_Up>
> +	friend _GLIBCXX26_CONSTEXPR __detail::__synth3way_t<_Up>
>  	operator<=>(const _Derived& __x, const _Derived& __y)
>  	{
>  	  return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
> @@ -888,11 +987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  							__detail::__synth3way);
>  	}
>  
> -      friend void
> +      friend _GLIBCXX26_CONSTEXPR void
>        swap(_Derived& __x, _Derived& __y) noexcept
>        { return __x.swap(__y); }
>  
>        template<typename _Predicate>
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	_M_erase_if(_Predicate __pred)
>  	{
> @@ -912,6 +1012,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        containers _M_cont;
>        [[no_unique_address]] _Compare _M_comp;
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_sort_uniq()
>        {
> @@ -921,13 +1022,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_unique();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_unique() requires (!_Multi)
>        {
>  	struct __key_equiv
>  	{
> +	  _GLIBCXX26_CONSTEXPR
>  	  __key_equiv(key_compare __c) : _M_comp(__c) { }
>  
> +	  _GLIBCXX26_CONSTEXPR
>  	  bool
>  	  operator()(const_reference __x, const_reference __y) const
>  	  { return !_M_comp(__x.first, __y.first) && !_M_comp(__y.first, __x.first); }
> @@ -958,12 +1062,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  				     ranges::__maybe_const_t<_Const, mapped_type>&>;
>        using difference_type   = ptrdiff_t;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator() = default;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Iterator<!_Const> __it) requires _Const
>        : _M_cont(__it._M_cont), _M_index(__it._M_index)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator*() const noexcept
>        {
> @@ -975,19 +1082,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	reference __p;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	const reference*
>  	operator->() const noexcept
>  	{ return std::__addressof(__p); }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        pointer
>        operator->() const
>        { return pointer{operator*()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator[](difference_type __n) const noexcept
>        { return *(*this + __n); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator++() noexcept
>        {
> @@ -995,6 +1106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator--() noexcept
>        {
> @@ -1002,6 +1114,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator++(int) noexcept
>        {
> @@ -1010,6 +1123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator--(int) noexcept
>        {
> @@ -1018,6 +1132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator+=(difference_type __n) noexcept
>        {
> @@ -1025,6 +1140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator-=(difference_type __n) noexcept
>        {
> @@ -1036,45 +1152,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend _Flat_map_impl;
>        friend _Iterator<!_Const>;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires (!_Const)
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(const _Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires _Const
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(difference_type __n, _Iterator __it) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator-(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it -= __n;
>  	return __it;
>        }
>  
> -      friend difference_type
> +      friend _GLIBCXX26_CONSTEXPR difference_type
>        operator-(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
>  	return __x._M_index - __y._M_index;
>        }
>  
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1082,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __x._M_index == __y._M_index;
>        }
>  
> -      friend strong_ordering
> +      friend _GLIBCXX26_CONSTEXPR strong_ordering
>        operator<=>(const _Iterator& __x, const _Iterator& __y)
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1146,30 +1264,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using _Impl::max_size;
>  
>        // element access
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](const key_type& __x)
>        { return try_emplace(__x).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](key_type&& __x)
>        { return try_emplace(std::move(__x)).first->second; }
>  
>        template<typename _Key2>
>  	requires __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	operator[](_Key2&& __x)
>  	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        at(const key_type& __x)
>        { return at<key_type>(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_type&
>        at(const key_type& __x) const
>        { return at<key_type>(__x); }
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	at(const _Key2& __x)
>  	{
> @@ -1181,6 +1305,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	const mapped_type&
>  	at(const _Key2& __x) const
>  	{
> @@ -1203,12 +1328,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(nullopt, __k, std::forward<_Args>(__args)...); }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(key_type&& __k, _Args&&... __args)
>  	{
> @@ -1222,6 +1349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<mapped_type, _Args...>
>  	  && (!is_convertible_v<_Key2&&, const_iterator>)
>  	  && (!is_convertible_v<_Key2&&, iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(_Key2&& __k, _Args&&... __args)
>  	{
> @@ -1231,12 +1359,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(__hint, __k, std::forward<_Args>(__args)...).first; }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
>  	{
> @@ -1248,6 +1378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	requires __transparent_comparator<_Compare>
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -1258,6 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(const key_type& __k, _Mapped&& __obj)
>  	{ return insert_or_assign<const key_type&, _Mapped>(__k, std::forward<_Mapped>(__obj)); }
> @@ -1265,6 +1397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1277,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(_Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1290,6 +1424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, const key_type& __k, _Mapped&& __obj)
>  	{
> @@ -1300,6 +1435,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1312,6 +1448,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, _Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1422,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> @@ -1590,6 +1728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> index 5bcda3464f1..04c2101aab1 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> @@ -17,7 +17,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -76,6 +76,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -127,6 +128,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(20,42) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -158,6 +160,7 @@ test02()
>    VERIFY( m.count(3) == 1 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -186,6 +189,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -218,6 +222,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -227,6 +232,7 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, -4, -5}) );
>  }
>  
> +constexpr
>  void
>  test06()
>  {
> @@ -243,6 +249,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -254,6 +261,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
>  }
>  
> +constexpr
>  void
>  test08()
>  {
> @@ -263,6 +271,7 @@ test08()
>    m[k] = 0;
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -274,8 +283,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -290,3 +299,31 @@ main()
>    test08();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test08();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> index f143f3acd1b..00644c14061 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> @@ -11,7 +11,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -74,6 +74,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -111,6 +112,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(12,3) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -137,6 +139,7 @@ test02()
>    VERIFY( m.count(3) == 2 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -165,6 +168,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -197,6 +201,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -205,6 +210,8 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::keys, (int[]){1, 2, 3, 3, 4, 5}) );
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, 3, -4, -5}) );
>  }
> +
> +constexpr
>  void
>  test06()
>  {
> @@ -221,6 +228,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -232,6 +240,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -243,8 +252,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -258,3 +267,30 @@ main()
>    test07();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> -- 
> 2.52.0
> 
> From c2a1c42266bdd87c97868735514b621cec3e30bd Mon Sep 17 00:00:00 2001
> From: Yuao Ma <c8ef@outlook.com>
> Date: Fri, 12 Dec 2025 19:26:01 +0800
> Subject: [PATCH] libstdc++: constexpr flat_map and flat_multimap
> 
> This patch makes flat_map and flat_multimap constexpr as part of P3372R3.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/version.def: Add FTM.
> 	* include/bits/version.h: Regenerate.
> 	* include/std/flat_map: Add constexpr.
> 	* testsuite/23_containers/flat_map/1.cc: Add constexpr test.
> 	* testsuite/23_containers/flat_multimap/1.cc: Add constexpr test.
> ---
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  10 ++
>  libstdc++-v3/include/std/flat_map             | 157 +++++++++++++++++-
>  .../testsuite/23_containers/flat_map/1.cc     |  43 ++++-
>  .../23_containers/flat_multimap/1.cc          |  42 ++++-
>  5 files changed, 245 insertions(+), 15 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> index 5b95db19ac6..f97276d80f2 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1351,6 +1351,14 @@ ftms = {
>    };
>  };
>  
> +ftms = {
> +  name = constexpr_flat_map;
> +  values = {
> +    v = 202502;
> +    cxxmin = 26;
> +  };
> +};
> +
>  ftms = {
>    name = constexpr_flat_set;
>    values = {
> diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> index eee9890a72b..97b37e9a62b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1496,6 +1496,16 @@
>  #endif /* !defined(__cpp_lib_constexpr_dynamic_alloc) */
>  #undef __glibcxx_want_constexpr_dynamic_alloc
>  
> +#if !defined(__cpp_lib_constexpr_flat_map)
> +# if (__cplusplus >  202302L)
> +#  define __glibcxx_constexpr_flat_map 202502L
> +#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_flat_map)
> +#   define __cpp_lib_constexpr_flat_map 202502L
> +#  endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_flat_map) */
> +#undef __glibcxx_want_constexpr_flat_map
> +
>  #if !defined(__cpp_lib_constexpr_flat_set)
>  # if (__cplusplus >  202302L)
>  #  define __glibcxx_constexpr_flat_set 202502L
> diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
> index 637c3b7fe0d..e270735a87b 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -33,6 +33,7 @@
>  #pragma GCC system_header
>  #endif
>  
> +#define __glibcxx_want_constexpr_flat_map
>  #define __glibcxx_want_flat_map
>  #include <bits/version.h>
>  
> @@ -108,9 +109,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        class value_compare
>        {
>  	[[no_unique_address]] key_compare _M_comp;
> +	_GLIBCXX26_CONSTEXPR
>  	value_compare(key_compare __c) : _M_comp(__c) { }
>  
>        public:
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	operator()(const_reference __x, const_reference __y) const
>  	{ return _M_comp(__x.first, __y.first); }
> @@ -129,10 +132,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	containers* _M_cont;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	_ClearGuard(containers& __cont)
>  	: _M_cont(std::__addressof(__cont))
>  	{ }
>  
> +	_GLIBCXX26_CONSTEXPR
>  	~_ClearGuard()
>  	{
>  	  if (_M_cont)
> @@ -142,24 +147,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    }
>  	}
>  
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_disable()
>  	{ _M_cont = nullptr; }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        _ClearGuard
>        _M_make_clear_guard()
>        { return _ClearGuard{this->_M_cont}; }
>  
>      public:
>        // constructors
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl() : _Flat_map_impl(key_compare()) { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        explicit
>        _Flat_map_impl(const key_compare& __comp)
>        : _M_cont(), _M_comp(__comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
>  		     const key_compare& __comp = key_compare())
> @@ -169,6 +179,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	_M_sort_uniq();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t,
>  		     key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
> @@ -180,12 +191,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
>  	: _M_cont(), _M_comp(__comp)
>  	{ insert(__first, __last); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
> @@ -193,20 +206,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ insert(__s, __first, __last); }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare())
>  	{ }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp)
>  	: _Flat_map_impl(__comp)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
>        : _Flat_map_impl(__il.begin(), __il.end(), __comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t __s,
>  		     initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
> @@ -216,12 +233,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // constructors with allocators
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	explicit
>  	_Flat_map_impl(const _Alloc& __a)
>  	: _Flat_map_impl(key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_compare& __comp, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a)),
> @@ -229,6 +248,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const _Alloc& __a)
> @@ -236,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const key_compare& __comp,
> @@ -249,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -257,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -271,6 +294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const _Derived& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a, __x._M_cont.keys),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a, __x._M_cont.values)),
> @@ -278,6 +302,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_Derived&& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>
>  		  (__a, std::move(__x._M_cont.keys)),
> @@ -288,6 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(std::move(__first), std::move(__last), key_compare(), __a)
> @@ -295,6 +321,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -303,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
> @@ -311,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
> @@ -320,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare(), __a)
> @@ -327,18 +357,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__comp, __a)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__il, key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -346,6 +379,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const _Alloc& __a)
> @@ -353,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const key_compare& __comp,
> @@ -360,6 +395,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	: _Flat_map_impl(__s, __il.begin(), __il.end(), __comp, __a)
>  	{ }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Derived&
>        operator=(initializer_list<value_type> __il)
>        {
> @@ -369,64 +405,79 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        // iterators
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        begin() noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        begin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        end() noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        end() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rbegin() noexcept
>        { return reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rbegin() const noexcept
>        { return const_reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rend() noexcept
>        { return reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rend() const noexcept
>        { return const_reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cbegin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cend() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crbegin() const noexcept
>        { return const_reverse_iterator(cend()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crend() const noexcept
>        { return const_reverse_iterator(cbegin()); }
>  
>        // capacity
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        empty() const noexcept
>        { return _M_cont.keys.empty(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        size() const noexcept
>        { return _M_cont.keys.size(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        max_size() const noexcept
>        { return std::min<size_type>(keys().max_size(), values().max_size()); }
> @@ -436,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // modifiers
>        template<typename _Key2, typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	_M_try_emplace(optional<const_iterator> __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -491,6 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<value_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	emplace(_Args&&... __args)
>  	{
> @@ -503,6 +556,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	emplace_hint(const_iterator __position, _Args&&... __args)
>  	{
> @@ -510,36 +564,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return _M_try_emplace(__position, std::move(__p.first), std::move(__p.second)).first;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(const value_type& __x)
>        { return emplace(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(value_type&& __x)
>        { return emplace(std::move(__x)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, const value_type& __x)
>        { return emplace_hint(__position, __x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, value_type&& __x)
>        { return emplace_hint(__position, std::move(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	insert(_Arg&& __x)
>  	{ return emplace(std::forward<_Arg>(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert(const_iterator __position, _Arg&& __x)
>  	{ return emplace_hint(__position, std::forward<_Arg>(__x)); }
>  
>      private:
>        template<typename _Iter, typename _Sent>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_insert(_Iter __first, _Sent __last)
>  	{
> @@ -577,11 +638,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>      public:
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(_InputIterator __first, _InputIterator __last)
>  	{ _M_insert(std::move(__first), std::move(__last)); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(__sorted_t, _InputIterator __first, _InputIterator __last)
>  	{
> @@ -590,18 +653,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert_range(_Rg&& __rg)
>  	{ _M_insert(ranges::begin(__rg), ranges::end(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(initializer_list<value_type> __il)
>        { insert(__il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(__sorted_t __s, initializer_list<value_type> __il)
>        { insert(__s, __il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        containers
>        extract() &&
>        {
> @@ -609,6 +676,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return {std::move(_M_cont.keys), std::move(_M_cont.values)};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont)
>        {
> @@ -622,10 +690,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // try_emplace and insert_or_assign defined directly in class flat_map only.
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(iterator __position)
>        { return erase(static_cast<const_iterator>(__position)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __position)
>        {
> @@ -637,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        erase(const key_type& __x)
>        { return erase<const key_type&>(__x); }
> @@ -646,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  || (__transparent_comparator<_Compare>
>  	      && !is_convertible_v<_Key2, iterator>
>  	      && !is_convertible_v<_Key2, const_iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	erase(_Key2&& __x)
>  	{
> @@ -655,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return __n;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __first, const_iterator __last)
>        {
> @@ -667,6 +740,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        swap(_Derived& __y) noexcept
>        {
> @@ -675,6 +749,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	ranges::swap(_M_cont.values, __y._M_cont.values);
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        clear() noexcept
>        {
> @@ -684,32 +759,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // observers
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        key_compare
>        key_comp() const
>        { return _M_comp; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        value_compare
>        value_comp() const
>        { return value_compare(_M_comp); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const key_container_type&
>        keys() const noexcept
>        { return _M_cont.keys; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_container_type&
>        values() const noexcept
>        { return _M_cont.values; }
>  
>        // map operations
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        find(const key_type& __x)
>        { return find<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        find(const key_type& __x) const
>        { return find<key_type>(__x); }
> @@ -717,6 +798,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	find(const _Key2& __x)
>  	{
> @@ -730,6 +812,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	find(const _Key2& __x) const
>  	{
> @@ -741,6 +824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        count(const key_type& __x) const
>        { return count<key_type>(__x); }
> @@ -748,6 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	count(const _Key2& __x) const
>  	{
> @@ -761,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        contains(const key_type& __x) const
>        { return contains<key_type>(__x); }
> @@ -768,16 +854,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	contains(const _Key2& __x) const
>  	{ return find(__x) != cend(); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        lower_bound(const key_type& __x)
>        { return lower_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        lower_bound(const key_type& __x) const
>        { return lower_bound<key_type>(__x); }
> @@ -785,6 +874,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	lower_bound(const _Key2& __x)
>  	{
> @@ -796,6 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	lower_bound(const _Key2& __x) const
>  	{
> @@ -805,11 +896,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        upper_bound(const key_type& __x)
>        { return upper_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        upper_bound(const key_type& __x) const
>        { return upper_bound<key_type>(__x); }
> @@ -817,6 +910,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	upper_bound(const _Key2& __x)
>  	{
> @@ -828,6 +922,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	upper_bound(const _Key2& __x) const
>  	{
> @@ -837,11 +932,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<iterator, iterator>
>        equal_range(const key_type& __x)
>        { return equal_range<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<const_iterator, const_iterator>
>        equal_range(const key_type& __x) const
>        { return equal_range<key_type>(__x); }
> @@ -849,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, iterator>
>  	equal_range(const _Key2& __x)
>  	{
> @@ -861,6 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<const_iterator, const_iterator>
>  	equal_range(const _Key2& __x) const
>  	{
> @@ -871,7 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Derived& __x, const _Derived& __y)
>        {
>  	return __x._M_cont.keys == __y._M_cont.keys
> @@ -880,7 +979,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Up = value_type>
>  	[[nodiscard]]
> -	friend __detail::__synth3way_t<_Up>
> +	friend _GLIBCXX26_CONSTEXPR __detail::__synth3way_t<_Up>
>  	operator<=>(const _Derived& __x, const _Derived& __y)
>  	{
>  	  return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
> @@ -888,11 +987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  							__detail::__synth3way);
>  	}
>  
> -      friend void
> +      friend _GLIBCXX26_CONSTEXPR void
>        swap(_Derived& __x, _Derived& __y) noexcept
>        { return __x.swap(__y); }
>  
>        template<typename _Predicate>
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	_M_erase_if(_Predicate __pred)
>  	{
> @@ -912,6 +1012,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        containers _M_cont;
>        [[no_unique_address]] _Compare _M_comp;
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_sort_uniq()
>        {
> @@ -921,13 +1022,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_unique();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_unique() requires (!_Multi)
>        {
>  	struct __key_equiv
>  	{
> +	  _GLIBCXX26_CONSTEXPR
>  	  __key_equiv(key_compare __c) : _M_comp(__c) { }
>  
> +	  _GLIBCXX26_CONSTEXPR
>  	  bool
>  	  operator()(const_reference __x, const_reference __y) const
>  	  { return !_M_comp(__x.first, __y.first) && !_M_comp(__y.first, __x.first); }
> @@ -958,12 +1062,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  				     ranges::__maybe_const_t<_Const, mapped_type>&>;
>        using difference_type   = ptrdiff_t;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator() = default;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Iterator<!_Const> __it) requires _Const
>        : _M_cont(__it._M_cont), _M_index(__it._M_index)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator*() const noexcept
>        {
> @@ -975,19 +1082,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	reference __p;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	const reference*
>  	operator->() const noexcept
>  	{ return std::__addressof(__p); }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        pointer
>        operator->() const
>        { return pointer{operator*()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator[](difference_type __n) const noexcept
>        { return *(*this + __n); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator++() noexcept
>        {
> @@ -995,6 +1106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator--() noexcept
>        {
> @@ -1002,6 +1114,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator++(int) noexcept
>        {
> @@ -1010,6 +1123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator--(int) noexcept
>        {
> @@ -1018,6 +1132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator+=(difference_type __n) noexcept
>        {
> @@ -1025,6 +1140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator-=(difference_type __n) noexcept
>        {
> @@ -1036,45 +1152,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend _Flat_map_impl;
>        friend _Iterator<!_Const>;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires (!_Const)
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(const _Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires _Const
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(difference_type __n, _Iterator __it) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator-(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it -= __n;
>  	return __it;
>        }
>  
> -      friend difference_type
> +      friend _GLIBCXX26_CONSTEXPR difference_type
>        operator-(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
>  	return __x._M_index - __y._M_index;
>        }
>  
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1082,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __x._M_index == __y._M_index;
>        }
>  
> -      friend strong_ordering
> +      friend _GLIBCXX26_CONSTEXPR strong_ordering
>        operator<=>(const _Iterator& __x, const _Iterator& __y)
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1146,30 +1264,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using _Impl::max_size;
>  
>        // element access
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](const key_type& __x)
>        { return try_emplace(__x).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](key_type&& __x)
>        { return try_emplace(std::move(__x)).first->second; }
>  
>        template<typename _Key2>
>  	requires __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	operator[](_Key2&& __x)
>  	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        at(const key_type& __x)
>        { return at<key_type>(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_type&
>        at(const key_type& __x) const
>        { return at<key_type>(__x); }
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	at(const _Key2& __x)
>  	{
> @@ -1181,6 +1305,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	const mapped_type&
>  	at(const _Key2& __x) const
>  	{
> @@ -1203,12 +1328,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(nullopt, __k, std::forward<_Args>(__args)...); }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(key_type&& __k, _Args&&... __args)
>  	{
> @@ -1222,6 +1349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<mapped_type, _Args...>
>  	  && (!is_convertible_v<_Key2&&, const_iterator>)
>  	  && (!is_convertible_v<_Key2&&, iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(_Key2&& __k, _Args&&... __args)
>  	{
> @@ -1231,12 +1359,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(__hint, __k, std::forward<_Args>(__args)...).first; }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
>  	{
> @@ -1248,6 +1378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	requires __transparent_comparator<_Compare>
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -1258,6 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(const key_type& __k, _Mapped&& __obj)
>  	{ return insert_or_assign<const key_type&, _Mapped>(__k, std::forward<_Mapped>(__obj)); }
> @@ -1265,6 +1397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1277,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(_Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1290,6 +1424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, const key_type& __k, _Mapped&& __obj)
>  	{
> @@ -1300,6 +1435,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1312,6 +1448,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, _Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1422,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> @@ -1590,6 +1728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> index 5bcda3464f1..04c2101aab1 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> @@ -17,7 +17,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -76,6 +76,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -127,6 +128,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(20,42) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -158,6 +160,7 @@ test02()
>    VERIFY( m.count(3) == 1 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -186,6 +189,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -218,6 +222,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -227,6 +232,7 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, -4, -5}) );
>  }
>  
> +constexpr
>  void
>  test06()
>  {
> @@ -243,6 +249,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -254,6 +261,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
>  }
>  
> +constexpr
>  void
>  test08()
>  {
> @@ -263,6 +271,7 @@ test08()
>    m[k] = 0;
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -274,8 +283,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -290,3 +299,31 @@ main()
>    test08();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test08();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> index f143f3acd1b..00644c14061 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> @@ -11,7 +11,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -74,6 +74,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -111,6 +112,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(12,3) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -137,6 +139,7 @@ test02()
>    VERIFY( m.count(3) == 2 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -165,6 +168,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -197,6 +201,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -205,6 +210,8 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::keys, (int[]){1, 2, 3, 3, 4, 5}) );
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, 3, -4, -5}) );
>  }
> +
> +constexpr
>  void
>  test06()
>  {
> @@ -221,6 +228,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -232,6 +240,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -243,8 +252,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -258,3 +267,30 @@ main()
>    test07();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> -- 
> 2.52.0
> 
> From c2a1c42266bdd87c97868735514b621cec3e30bd Mon Sep 17 00:00:00 2001
> From: Yuao Ma <c8ef@outlook.com>
> Date: Fri, 12 Dec 2025 19:26:01 +0800
> Subject: [PATCH] libstdc++: constexpr flat_map and flat_multimap
> 
> This patch makes flat_map and flat_multimap constexpr as part of P3372R3.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/version.def: Add FTM.
> 	* include/bits/version.h: Regenerate.
> 	* include/std/flat_map: Add constexpr.
> 	* testsuite/23_containers/flat_map/1.cc: Add constexpr test.
> 	* testsuite/23_containers/flat_multimap/1.cc: Add constexpr test.
> ---
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  10 ++
>  libstdc++-v3/include/std/flat_map             | 157 +++++++++++++++++-
>  .../testsuite/23_containers/flat_map/1.cc     |  43 ++++-
>  .../23_containers/flat_multimap/1.cc          |  42 ++++-
>  5 files changed, 245 insertions(+), 15 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> index 5b95db19ac6..f97276d80f2 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1351,6 +1351,14 @@ ftms = {
>    };
>  };
>  
> +ftms = {
> +  name = constexpr_flat_map;
> +  values = {
> +    v = 202502;
> +    cxxmin = 26;
> +  };
> +};
> +
>  ftms = {
>    name = constexpr_flat_set;
>    values = {
> diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> index eee9890a72b..97b37e9a62b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1496,6 +1496,16 @@
>  #endif /* !defined(__cpp_lib_constexpr_dynamic_alloc) */
>  #undef __glibcxx_want_constexpr_dynamic_alloc
>  
> +#if !defined(__cpp_lib_constexpr_flat_map)
> +# if (__cplusplus >  202302L)
> +#  define __glibcxx_constexpr_flat_map 202502L
> +#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_flat_map)
> +#   define __cpp_lib_constexpr_flat_map 202502L
> +#  endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_flat_map) */
> +#undef __glibcxx_want_constexpr_flat_map
> +
>  #if !defined(__cpp_lib_constexpr_flat_set)
>  # if (__cplusplus >  202302L)
>  #  define __glibcxx_constexpr_flat_set 202502L
> diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
> index 637c3b7fe0d..e270735a87b 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -33,6 +33,7 @@
>  #pragma GCC system_header
>  #endif
>  
> +#define __glibcxx_want_constexpr_flat_map
>  #define __glibcxx_want_flat_map
>  #include <bits/version.h>
>  
> @@ -108,9 +109,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        class value_compare
>        {
>  	[[no_unique_address]] key_compare _M_comp;
> +	_GLIBCXX26_CONSTEXPR
>  	value_compare(key_compare __c) : _M_comp(__c) { }
>  
>        public:
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	operator()(const_reference __x, const_reference __y) const
>  	{ return _M_comp(__x.first, __y.first); }
> @@ -129,10 +132,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	containers* _M_cont;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	_ClearGuard(containers& __cont)
>  	: _M_cont(std::__addressof(__cont))
>  	{ }
>  
> +	_GLIBCXX26_CONSTEXPR
>  	~_ClearGuard()
>  	{
>  	  if (_M_cont)
> @@ -142,24 +147,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    }
>  	}
>  
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_disable()
>  	{ _M_cont = nullptr; }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        _ClearGuard
>        _M_make_clear_guard()
>        { return _ClearGuard{this->_M_cont}; }
>  
>      public:
>        // constructors
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl() : _Flat_map_impl(key_compare()) { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        explicit
>        _Flat_map_impl(const key_compare& __comp)
>        : _M_cont(), _M_comp(__comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
>  		     const key_compare& __comp = key_compare())
> @@ -169,6 +179,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	_M_sort_uniq();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t,
>  		     key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
> @@ -180,12 +191,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
>  	: _M_cont(), _M_comp(__comp)
>  	{ insert(__first, __last); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
> @@ -193,20 +206,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ insert(__s, __first, __last); }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare())
>  	{ }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp)
>  	: _Flat_map_impl(__comp)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
>        : _Flat_map_impl(__il.begin(), __il.end(), __comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t __s,
>  		     initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
> @@ -216,12 +233,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // constructors with allocators
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	explicit
>  	_Flat_map_impl(const _Alloc& __a)
>  	: _Flat_map_impl(key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_compare& __comp, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a)),
> @@ -229,6 +248,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const _Alloc& __a)
> @@ -236,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const key_compare& __comp,
> @@ -249,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -257,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -271,6 +294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const _Derived& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a, __x._M_cont.keys),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a, __x._M_cont.values)),
> @@ -278,6 +302,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_Derived&& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>
>  		  (__a, std::move(__x._M_cont.keys)),
> @@ -288,6 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(std::move(__first), std::move(__last), key_compare(), __a)
> @@ -295,6 +321,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -303,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
> @@ -311,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
> @@ -320,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare(), __a)
> @@ -327,18 +357,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__comp, __a)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__il, key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -346,6 +379,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const _Alloc& __a)
> @@ -353,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const key_compare& __comp,
> @@ -360,6 +395,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	: _Flat_map_impl(__s, __il.begin(), __il.end(), __comp, __a)
>  	{ }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Derived&
>        operator=(initializer_list<value_type> __il)
>        {
> @@ -369,64 +405,79 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        // iterators
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        begin() noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        begin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        end() noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        end() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rbegin() noexcept
>        { return reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rbegin() const noexcept
>        { return const_reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rend() noexcept
>        { return reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rend() const noexcept
>        { return const_reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cbegin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cend() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crbegin() const noexcept
>        { return const_reverse_iterator(cend()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crend() const noexcept
>        { return const_reverse_iterator(cbegin()); }
>  
>        // capacity
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        empty() const noexcept
>        { return _M_cont.keys.empty(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        size() const noexcept
>        { return _M_cont.keys.size(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        max_size() const noexcept
>        { return std::min<size_type>(keys().max_size(), values().max_size()); }
> @@ -436,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // modifiers
>        template<typename _Key2, typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	_M_try_emplace(optional<const_iterator> __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -491,6 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<value_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	emplace(_Args&&... __args)
>  	{
> @@ -503,6 +556,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	emplace_hint(const_iterator __position, _Args&&... __args)
>  	{
> @@ -510,36 +564,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return _M_try_emplace(__position, std::move(__p.first), std::move(__p.second)).first;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(const value_type& __x)
>        { return emplace(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(value_type&& __x)
>        { return emplace(std::move(__x)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, const value_type& __x)
>        { return emplace_hint(__position, __x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, value_type&& __x)
>        { return emplace_hint(__position, std::move(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	insert(_Arg&& __x)
>  	{ return emplace(std::forward<_Arg>(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert(const_iterator __position, _Arg&& __x)
>  	{ return emplace_hint(__position, std::forward<_Arg>(__x)); }
>  
>      private:
>        template<typename _Iter, typename _Sent>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_insert(_Iter __first, _Sent __last)
>  	{
> @@ -577,11 +638,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>      public:
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(_InputIterator __first, _InputIterator __last)
>  	{ _M_insert(std::move(__first), std::move(__last)); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(__sorted_t, _InputIterator __first, _InputIterator __last)
>  	{
> @@ -590,18 +653,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert_range(_Rg&& __rg)
>  	{ _M_insert(ranges::begin(__rg), ranges::end(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(initializer_list<value_type> __il)
>        { insert(__il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(__sorted_t __s, initializer_list<value_type> __il)
>        { insert(__s, __il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        containers
>        extract() &&
>        {
> @@ -609,6 +676,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return {std::move(_M_cont.keys), std::move(_M_cont.values)};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont)
>        {
> @@ -622,10 +690,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // try_emplace and insert_or_assign defined directly in class flat_map only.
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(iterator __position)
>        { return erase(static_cast<const_iterator>(__position)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __position)
>        {
> @@ -637,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        erase(const key_type& __x)
>        { return erase<const key_type&>(__x); }
> @@ -646,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  || (__transparent_comparator<_Compare>
>  	      && !is_convertible_v<_Key2, iterator>
>  	      && !is_convertible_v<_Key2, const_iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	erase(_Key2&& __x)
>  	{
> @@ -655,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return __n;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __first, const_iterator __last)
>        {
> @@ -667,6 +740,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        swap(_Derived& __y) noexcept
>        {
> @@ -675,6 +749,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	ranges::swap(_M_cont.values, __y._M_cont.values);
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        clear() noexcept
>        {
> @@ -684,32 +759,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // observers
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        key_compare
>        key_comp() const
>        { return _M_comp; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        value_compare
>        value_comp() const
>        { return value_compare(_M_comp); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const key_container_type&
>        keys() const noexcept
>        { return _M_cont.keys; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_container_type&
>        values() const noexcept
>        { return _M_cont.values; }
>  
>        // map operations
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        find(const key_type& __x)
>        { return find<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        find(const key_type& __x) const
>        { return find<key_type>(__x); }
> @@ -717,6 +798,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	find(const _Key2& __x)
>  	{
> @@ -730,6 +812,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	find(const _Key2& __x) const
>  	{
> @@ -741,6 +824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        count(const key_type& __x) const
>        { return count<key_type>(__x); }
> @@ -748,6 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	count(const _Key2& __x) const
>  	{
> @@ -761,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        contains(const key_type& __x) const
>        { return contains<key_type>(__x); }
> @@ -768,16 +854,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	contains(const _Key2& __x) const
>  	{ return find(__x) != cend(); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        lower_bound(const key_type& __x)
>        { return lower_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        lower_bound(const key_type& __x) const
>        { return lower_bound<key_type>(__x); }
> @@ -785,6 +874,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	lower_bound(const _Key2& __x)
>  	{
> @@ -796,6 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	lower_bound(const _Key2& __x) const
>  	{
> @@ -805,11 +896,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        upper_bound(const key_type& __x)
>        { return upper_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        upper_bound(const key_type& __x) const
>        { return upper_bound<key_type>(__x); }
> @@ -817,6 +910,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	upper_bound(const _Key2& __x)
>  	{
> @@ -828,6 +922,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	upper_bound(const _Key2& __x) const
>  	{
> @@ -837,11 +932,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<iterator, iterator>
>        equal_range(const key_type& __x)
>        { return equal_range<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<const_iterator, const_iterator>
>        equal_range(const key_type& __x) const
>        { return equal_range<key_type>(__x); }
> @@ -849,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, iterator>
>  	equal_range(const _Key2& __x)
>  	{
> @@ -861,6 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<const_iterator, const_iterator>
>  	equal_range(const _Key2& __x) const
>  	{
> @@ -871,7 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Derived& __x, const _Derived& __y)
>        {
>  	return __x._M_cont.keys == __y._M_cont.keys
> @@ -880,7 +979,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Up = value_type>
>  	[[nodiscard]]
> -	friend __detail::__synth3way_t<_Up>
> +	friend _GLIBCXX26_CONSTEXPR __detail::__synth3way_t<_Up>
>  	operator<=>(const _Derived& __x, const _Derived& __y)
>  	{
>  	  return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
> @@ -888,11 +987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  							__detail::__synth3way);
>  	}
>  
> -      friend void
> +      friend _GLIBCXX26_CONSTEXPR void
>        swap(_Derived& __x, _Derived& __y) noexcept
>        { return __x.swap(__y); }
>  
>        template<typename _Predicate>
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	_M_erase_if(_Predicate __pred)
>  	{
> @@ -912,6 +1012,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        containers _M_cont;
>        [[no_unique_address]] _Compare _M_comp;
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_sort_uniq()
>        {
> @@ -921,13 +1022,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_unique();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_unique() requires (!_Multi)
>        {
>  	struct __key_equiv
>  	{
> +	  _GLIBCXX26_CONSTEXPR
>  	  __key_equiv(key_compare __c) : _M_comp(__c) { }
>  
> +	  _GLIBCXX26_CONSTEXPR
>  	  bool
>  	  operator()(const_reference __x, const_reference __y) const
>  	  { return !_M_comp(__x.first, __y.first) && !_M_comp(__y.first, __x.first); }
> @@ -958,12 +1062,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  				     ranges::__maybe_const_t<_Const, mapped_type>&>;
>        using difference_type   = ptrdiff_t;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator() = default;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Iterator<!_Const> __it) requires _Const
>        : _M_cont(__it._M_cont), _M_index(__it._M_index)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator*() const noexcept
>        {
> @@ -975,19 +1082,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	reference __p;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	const reference*
>  	operator->() const noexcept
>  	{ return std::__addressof(__p); }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        pointer
>        operator->() const
>        { return pointer{operator*()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator[](difference_type __n) const noexcept
>        { return *(*this + __n); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator++() noexcept
>        {
> @@ -995,6 +1106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator--() noexcept
>        {
> @@ -1002,6 +1114,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator++(int) noexcept
>        {
> @@ -1010,6 +1123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator--(int) noexcept
>        {
> @@ -1018,6 +1132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator+=(difference_type __n) noexcept
>        {
> @@ -1025,6 +1140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator-=(difference_type __n) noexcept
>        {
> @@ -1036,45 +1152,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend _Flat_map_impl;
>        friend _Iterator<!_Const>;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires (!_Const)
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(const _Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires _Const
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(difference_type __n, _Iterator __it) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator-(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it -= __n;
>  	return __it;
>        }
>  
> -      friend difference_type
> +      friend _GLIBCXX26_CONSTEXPR difference_type
>        operator-(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
>  	return __x._M_index - __y._M_index;
>        }
>  
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1082,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __x._M_index == __y._M_index;
>        }
>  
> -      friend strong_ordering
> +      friend _GLIBCXX26_CONSTEXPR strong_ordering
>        operator<=>(const _Iterator& __x, const _Iterator& __y)
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1146,30 +1264,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using _Impl::max_size;
>  
>        // element access
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](const key_type& __x)
>        { return try_emplace(__x).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](key_type&& __x)
>        { return try_emplace(std::move(__x)).first->second; }
>  
>        template<typename _Key2>
>  	requires __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	operator[](_Key2&& __x)
>  	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        at(const key_type& __x)
>        { return at<key_type>(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_type&
>        at(const key_type& __x) const
>        { return at<key_type>(__x); }
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	at(const _Key2& __x)
>  	{
> @@ -1181,6 +1305,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	const mapped_type&
>  	at(const _Key2& __x) const
>  	{
> @@ -1203,12 +1328,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(nullopt, __k, std::forward<_Args>(__args)...); }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(key_type&& __k, _Args&&... __args)
>  	{
> @@ -1222,6 +1349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<mapped_type, _Args...>
>  	  && (!is_convertible_v<_Key2&&, const_iterator>)
>  	  && (!is_convertible_v<_Key2&&, iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(_Key2&& __k, _Args&&... __args)
>  	{
> @@ -1231,12 +1359,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(__hint, __k, std::forward<_Args>(__args)...).first; }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
>  	{
> @@ -1248,6 +1378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	requires __transparent_comparator<_Compare>
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -1258,6 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(const key_type& __k, _Mapped&& __obj)
>  	{ return insert_or_assign<const key_type&, _Mapped>(__k, std::forward<_Mapped>(__obj)); }
> @@ -1265,6 +1397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1277,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(_Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1290,6 +1424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, const key_type& __k, _Mapped&& __obj)
>  	{
> @@ -1300,6 +1435,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1312,6 +1448,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, _Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1422,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> @@ -1590,6 +1728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> index 5bcda3464f1..04c2101aab1 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> @@ -17,7 +17,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -76,6 +76,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -127,6 +128,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(20,42) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -158,6 +160,7 @@ test02()
>    VERIFY( m.count(3) == 1 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -186,6 +189,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -218,6 +222,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -227,6 +232,7 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, -4, -5}) );
>  }
>  
> +constexpr
>  void
>  test06()
>  {
> @@ -243,6 +249,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -254,6 +261,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
>  }
>  
> +constexpr
>  void
>  test08()
>  {
> @@ -263,6 +271,7 @@ test08()
>    m[k] = 0;
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -274,8 +283,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -290,3 +299,31 @@ main()
>    test08();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test08();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> index f143f3acd1b..00644c14061 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> @@ -11,7 +11,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -74,6 +74,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -111,6 +112,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(12,3) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -137,6 +139,7 @@ test02()
>    VERIFY( m.count(3) == 2 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -165,6 +168,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -197,6 +201,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -205,6 +210,8 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::keys, (int[]){1, 2, 3, 3, 4, 5}) );
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, 3, -4, -5}) );
>  }
> +
> +constexpr
>  void
>  test06()
>  {
> @@ -221,6 +228,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -232,6 +240,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -243,8 +252,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -258,3 +267,30 @@ main()
>    test07();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> -- 
> 2.52.0
> 
> From c2a1c42266bdd87c97868735514b621cec3e30bd Mon Sep 17 00:00:00 2001
> From: Yuao Ma <c8ef@outlook.com>
> Date: Fri, 12 Dec 2025 19:26:01 +0800
> Subject: [PATCH] libstdc++: constexpr flat_map and flat_multimap
> 
> This patch makes flat_map and flat_multimap constexpr as part of P3372R3.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/version.def: Add FTM.
> 	* include/bits/version.h: Regenerate.
> 	* include/std/flat_map: Add constexpr.
> 	* testsuite/23_containers/flat_map/1.cc: Add constexpr test.
> 	* testsuite/23_containers/flat_multimap/1.cc: Add constexpr test.
> ---
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  10 ++
>  libstdc++-v3/include/std/flat_map             | 157 +++++++++++++++++-
>  .../testsuite/23_containers/flat_map/1.cc     |  43 ++++-
>  .../23_containers/flat_multimap/1.cc          |  42 ++++-
>  5 files changed, 245 insertions(+), 15 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
> index 5b95db19ac6..f97276d80f2 100644
> --- a/libstdc++-v3/include/bits/version.def
> +++ b/libstdc++-v3/include/bits/version.def
> @@ -1351,6 +1351,14 @@ ftms = {
>    };
>  };
>  
> +ftms = {
> +  name = constexpr_flat_map;
> +  values = {
> +    v = 202502;
> +    cxxmin = 26;
> +  };
> +};
> +
>  ftms = {
>    name = constexpr_flat_set;
>    values = {
> diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
> index eee9890a72b..97b37e9a62b 100644
> --- a/libstdc++-v3/include/bits/version.h
> +++ b/libstdc++-v3/include/bits/version.h
> @@ -1496,6 +1496,16 @@
>  #endif /* !defined(__cpp_lib_constexpr_dynamic_alloc) */
>  #undef __glibcxx_want_constexpr_dynamic_alloc
>  
> +#if !defined(__cpp_lib_constexpr_flat_map)
> +# if (__cplusplus >  202302L)
> +#  define __glibcxx_constexpr_flat_map 202502L
> +#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_flat_map)
> +#   define __cpp_lib_constexpr_flat_map 202502L
> +#  endif
> +# endif
> +#endif /* !defined(__cpp_lib_constexpr_flat_map) */
> +#undef __glibcxx_want_constexpr_flat_map
> +
>  #if !defined(__cpp_lib_constexpr_flat_set)
>  # if (__cplusplus >  202302L)
>  #  define __glibcxx_constexpr_flat_set 202502L
> diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
> index 637c3b7fe0d..e270735a87b 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -33,6 +33,7 @@
>  #pragma GCC system_header
>  #endif
>  
> +#define __glibcxx_want_constexpr_flat_map
>  #define __glibcxx_want_flat_map
>  #include <bits/version.h>
>  
> @@ -108,9 +109,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        class value_compare
>        {
>  	[[no_unique_address]] key_compare _M_comp;
> +	_GLIBCXX26_CONSTEXPR
>  	value_compare(key_compare __c) : _M_comp(__c) { }
>  
>        public:
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	operator()(const_reference __x, const_reference __y) const
>  	{ return _M_comp(__x.first, __y.first); }
> @@ -129,10 +132,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	containers* _M_cont;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	_ClearGuard(containers& __cont)
>  	: _M_cont(std::__addressof(__cont))
>  	{ }
>  
> +	_GLIBCXX26_CONSTEXPR
>  	~_ClearGuard()
>  	{
>  	  if (_M_cont)
> @@ -142,24 +147,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    }
>  	}
>  
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_disable()
>  	{ _M_cont = nullptr; }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        _ClearGuard
>        _M_make_clear_guard()
>        { return _ClearGuard{this->_M_cont}; }
>  
>      public:
>        // constructors
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl() : _Flat_map_impl(key_compare()) { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        explicit
>        _Flat_map_impl(const key_compare& __comp)
>        : _M_cont(), _M_comp(__comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
>  		     const key_compare& __comp = key_compare())
> @@ -169,6 +179,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	_M_sort_uniq();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t,
>  		     key_container_type __key_cont,
>  		     mapped_container_type __mapped_cont,
> @@ -180,12 +191,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
>  	: _M_cont(), _M_comp(__comp)
>  	{ insert(__first, __last); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp = key_compare())
> @@ -193,20 +206,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ insert(__s, __first, __last); }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare())
>  	{ }
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp)
>  	: _Flat_map_impl(__comp)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
>        : _Flat_map_impl(__il.begin(), __il.end(), __comp)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Flat_map_impl(__sorted_t __s,
>  		     initializer_list<value_type> __il,
>  		     const key_compare& __comp = key_compare())
> @@ -216,12 +233,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // constructors with allocators
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	explicit
>  	_Flat_map_impl(const _Alloc& __a)
>  	: _Flat_map_impl(key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_compare& __comp, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a)),
> @@ -229,6 +248,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const _Alloc& __a)
> @@ -236,6 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
>  		       const key_compare& __comp,
> @@ -249,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -257,6 +279,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t,
>  		       const key_container_type& __key_cont,
>  		       const mapped_container_type& __mapped_cont,
> @@ -271,6 +294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(const _Derived& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>(__a, __x._M_cont.keys),
>  		  std::make_obj_using_allocator<mapped_container_type>(__a, __x._M_cont.values)),
> @@ -278,6 +302,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_Derived&& __x, const _Alloc& __a)
>  	: _M_cont(std::make_obj_using_allocator<key_container_type>
>  		  (__a, std::move(__x._M_cont.keys)),
> @@ -288,6 +313,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(std::move(__first), std::move(__last), key_compare(), __a)
> @@ -295,6 +321,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(_InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -303,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const _Alloc& __a)
> @@ -311,6 +339,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__has_input_iter_cat _InputIterator,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       _InputIterator __first, _InputIterator __last,
>  		       const key_compare& __comp,
> @@ -320,6 +349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(from_range, std::forward<_Rg>(__rg), key_compare(), __a)
> @@ -327,18 +357,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<__detail::__container_compatible_range<value_type> _Rg,
>  	       __allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(from_range_t, _Rg&& __rg, const key_compare& __comp,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__comp, __a)
>  	{ insert_range(std::forward<_Rg>(__rg)); }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const _Alloc& __a)
>  	: _Flat_map_impl(__il, key_compare(), __a)
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(initializer_list<value_type> __il,
>  		       const key_compare& __comp,
>  		       const _Alloc& __a)
> @@ -346,6 +379,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const _Alloc& __a)
> @@ -353,6 +387,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ }
>  
>        template<__allocator_for<key_container_type, mapped_container_type> _Alloc>
> +	_GLIBCXX26_CONSTEXPR
>  	_Flat_map_impl(__sorted_t __s,
>  		       initializer_list<value_type> __il,
>  		       const key_compare& __comp,
> @@ -360,6 +395,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	: _Flat_map_impl(__s, __il.begin(), __il.end(), __comp, __a)
>  	{ }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Derived&
>        operator=(initializer_list<value_type> __il)
>        {
> @@ -369,64 +405,79 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>  
>        // iterators
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        begin() noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        begin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        end() noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        end() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rbegin() noexcept
>        { return reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rbegin() const noexcept
>        { return const_reverse_iterator(end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reverse_iterator
>        rend() noexcept
>        { return reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        rend() const noexcept
>        { return const_reverse_iterator(begin()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cbegin() const noexcept
>        { return {this, _M_cont.keys.cbegin()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        cend() const noexcept
>        { return {this, _M_cont.keys.cend()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crbegin() const noexcept
>        { return const_reverse_iterator(cend()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const_reverse_iterator
>        crend() const noexcept
>        { return const_reverse_iterator(cbegin()); }
>  
>        // capacity
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        empty() const noexcept
>        { return _M_cont.keys.empty(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        size() const noexcept
>        { return _M_cont.keys.size(); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        max_size() const noexcept
>        { return std::min<size_type>(keys().max_size(), values().max_size()); }
> @@ -436,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // modifiers
>        template<typename _Key2, typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	_M_try_emplace(optional<const_iterator> __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -491,6 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<value_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	emplace(_Args&&... __args)
>  	{
> @@ -503,6 +556,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<typename... _Args>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	emplace_hint(const_iterator __position, _Args&&... __args)
>  	{
> @@ -510,36 +564,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return _M_try_emplace(__position, std::move(__p.first), std::move(__p.second)).first;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(const value_type& __x)
>        { return emplace(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        __emplace_result_t
>        insert(value_type&& __x)
>        { return emplace(std::move(__x)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, const value_type& __x)
>        { return emplace_hint(__position, __x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        insert(const_iterator __position, value_type&& __x)
>        { return emplace_hint(__position, std::move(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	__emplace_result_t
>  	insert(_Arg&& __x)
>  	{ return emplace(std::forward<_Arg>(__x)); }
>  
>        template<typename _Arg>
>  	requires is_constructible_v<value_type, _Arg>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert(const_iterator __position, _Arg&& __x)
>  	{ return emplace_hint(__position, std::forward<_Arg>(__x)); }
>  
>      private:
>        template<typename _Iter, typename _Sent>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	_M_insert(_Iter __first, _Sent __last)
>  	{
> @@ -577,11 +638,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>      public:
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(_InputIterator __first, _InputIterator __last)
>  	{ _M_insert(std::move(__first), std::move(__last)); }
>  
>        template<__has_input_iter_cat _InputIterator>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert(__sorted_t, _InputIterator __first, _InputIterator __last)
>  	{
> @@ -590,18 +653,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        template<__detail::__container_compatible_range<value_type> _Rg>
> +	_GLIBCXX26_CONSTEXPR
>  	void
>  	insert_range(_Rg&& __rg)
>  	{ _M_insert(ranges::begin(__rg), ranges::end(__rg)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(initializer_list<value_type> __il)
>        { insert(__il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        insert(__sorted_t __s, initializer_list<value_type> __il)
>        { insert(__s, __il.begin(), __il.end()); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        containers
>        extract() &&
>        {
> @@ -609,6 +676,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return {std::move(_M_cont.keys), std::move(_M_cont.values)};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont)
>        {
> @@ -622,10 +690,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // try_emplace and insert_or_assign defined directly in class flat_map only.
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(iterator __position)
>        { return erase(static_cast<const_iterator>(__position)); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __position)
>        {
> @@ -637,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        erase(const key_type& __x)
>        { return erase<const key_type&>(__x); }
> @@ -646,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  || (__transparent_comparator<_Compare>
>  	      && !is_convertible_v<_Key2, iterator>
>  	      && !is_convertible_v<_Key2, const_iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	erase(_Key2&& __x)
>  	{
> @@ -655,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  return __n;
>  	}
>  
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        erase(const_iterator __first, const_iterator __last)
>        {
> @@ -667,6 +740,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return iterator{this, __it};
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        swap(_Derived& __y) noexcept
>        {
> @@ -675,6 +749,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	ranges::swap(_M_cont.values, __y._M_cont.values);
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        clear() noexcept
>        {
> @@ -684,32 +759,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        // observers
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        key_compare
>        key_comp() const
>        { return _M_comp; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        value_compare
>        value_comp() const
>        { return value_compare(_M_comp); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const key_container_type&
>        keys() const noexcept
>        { return _M_cont.keys; }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_container_type&
>        values() const noexcept
>        { return _M_cont.values; }
>  
>        // map operations
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        find(const key_type& __x)
>        { return find<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        find(const key_type& __x) const
>        { return find<key_type>(__x); }
> @@ -717,6 +798,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	find(const _Key2& __x)
>  	{
> @@ -730,6 +812,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	find(const _Key2& __x) const
>  	{
> @@ -741,6 +824,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        size_type
>        count(const key_type& __x) const
>        { return count<key_type>(__x); }
> @@ -748,6 +832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	count(const _Key2& __x) const
>  	{
> @@ -761,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        bool
>        contains(const key_type& __x) const
>        { return contains<key_type>(__x); }
> @@ -768,16 +854,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	bool
>  	contains(const _Key2& __x) const
>  	{ return find(__x) != cend(); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        lower_bound(const key_type& __x)
>        { return lower_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        lower_bound(const key_type& __x) const
>        { return lower_bound<key_type>(__x); }
> @@ -785,6 +874,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	lower_bound(const _Key2& __x)
>  	{
> @@ -796,6 +886,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	lower_bound(const _Key2& __x) const
>  	{
> @@ -805,11 +896,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        iterator
>        upper_bound(const key_type& __x)
>        { return upper_bound<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        const_iterator
>        upper_bound(const key_type& __x) const
>        { return upper_bound<key_type>(__x); }
> @@ -817,6 +910,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	upper_bound(const _Key2& __x)
>  	{
> @@ -828,6 +922,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	const_iterator
>  	upper_bound(const _Key2& __x) const
>  	{
> @@ -837,11 +932,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<iterator, iterator>
>        equal_range(const key_type& __x)
>        { return equal_range<key_type>(__x); }
>  
>        [[nodiscard]]
> +      _GLIBCXX26_CONSTEXPR
>        pair<const_iterator, const_iterator>
>        equal_range(const key_type& __x) const
>        { return equal_range<key_type>(__x); }
> @@ -849,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, iterator>
>  	equal_range(const _Key2& __x)
>  	{
> @@ -861,6 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
>  	[[nodiscard]]
> +	_GLIBCXX26_CONSTEXPR
>  	pair<const_iterator, const_iterator>
>  	equal_range(const _Key2& __x) const
>  	{
> @@ -871,7 +970,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	}
>  
>        [[nodiscard]]
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Derived& __x, const _Derived& __y)
>        {
>  	return __x._M_cont.keys == __y._M_cont.keys
> @@ -880,7 +979,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Up = value_type>
>  	[[nodiscard]]
> -	friend __detail::__synth3way_t<_Up>
> +	friend _GLIBCXX26_CONSTEXPR __detail::__synth3way_t<_Up>
>  	operator<=>(const _Derived& __x, const _Derived& __y)
>  	{
>  	  return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
> @@ -888,11 +987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  							__detail::__synth3way);
>  	}
>  
> -      friend void
> +      friend _GLIBCXX26_CONSTEXPR void
>        swap(_Derived& __x, _Derived& __y) noexcept
>        { return __x.swap(__y); }
>  
>        template<typename _Predicate>
> +	_GLIBCXX26_CONSTEXPR
>  	size_type
>  	_M_erase_if(_Predicate __pred)
>  	{
> @@ -912,6 +1012,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        containers _M_cont;
>        [[no_unique_address]] _Compare _M_comp;
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_sort_uniq()
>        {
> @@ -921,13 +1022,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_unique();
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        void
>        _M_unique() requires (!_Multi)
>        {
>  	struct __key_equiv
>  	{
> +	  _GLIBCXX26_CONSTEXPR
>  	  __key_equiv(key_compare __c) : _M_comp(__c) { }
>  
> +	  _GLIBCXX26_CONSTEXPR
>  	  bool
>  	  operator()(const_reference __x, const_reference __y) const
>  	  { return !_M_comp(__x.first, __y.first) && !_M_comp(__y.first, __x.first); }
> @@ -958,12 +1062,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  				     ranges::__maybe_const_t<_Const, mapped_type>&>;
>        using difference_type   = ptrdiff_t;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator() = default;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Iterator<!_Const> __it) requires _Const
>        : _M_cont(__it._M_cont), _M_index(__it._M_index)
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator*() const noexcept
>        {
> @@ -975,19 +1082,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        {
>  	reference __p;
>  
> +	_GLIBCXX26_CONSTEXPR
>  	const reference*
>  	operator->() const noexcept
>  	{ return std::__addressof(__p); }
>        };
>  
> +      _GLIBCXX26_CONSTEXPR
>        pointer
>        operator->() const
>        { return pointer{operator*()}; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        reference
>        operator[](difference_type __n) const noexcept
>        { return *(*this + __n); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator++() noexcept
>        {
> @@ -995,6 +1106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator--() noexcept
>        {
> @@ -1002,6 +1114,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator++(int) noexcept
>        {
> @@ -1010,6 +1123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator
>        operator--(int) noexcept
>        {
> @@ -1018,6 +1132,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __tmp;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator+=(difference_type __n) noexcept
>        {
> @@ -1025,6 +1140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return *this;
>        }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator&
>        operator-=(difference_type __n) noexcept
>        {
> @@ -1036,45 +1152,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend _Flat_map_impl;
>        friend _Iterator<!_Const>;
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(_Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires (!_Const)
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> +      _GLIBCXX26_CONSTEXPR
>        _Iterator(const _Flat_map_impl* __fm, typename key_container_type::const_iterator __it)
>        requires _Const
>        : _M_cont(std::__addressof(__fm->_M_cont)), _M_index(__it - __fm->keys().cbegin())
>        { }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator+(difference_type __n, _Iterator __it) noexcept
>        {
>  	__it += __n;
>  	return __it;
>        }
>  
> -      friend _Iterator
> +      friend _GLIBCXX26_CONSTEXPR _Iterator
>        operator-(_Iterator __it, difference_type __n) noexcept
>        {
>  	__it -= __n;
>  	return __it;
>        }
>  
> -      friend difference_type
> +      friend _GLIBCXX26_CONSTEXPR difference_type
>        operator-(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
>  	return __x._M_index - __y._M_index;
>        }
>  
> -      friend bool
> +      friend _GLIBCXX26_CONSTEXPR bool
>        operator==(const _Iterator& __x, const _Iterator& __y) noexcept
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1082,7 +1200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	return __x._M_index == __y._M_index;
>        }
>  
> -      friend strong_ordering
> +      friend _GLIBCXX26_CONSTEXPR strong_ordering
>        operator<=>(const _Iterator& __x, const _Iterator& __y)
>        {
>  	__glibcxx_assert(__x._M_cont == __y._M_cont);
> @@ -1146,30 +1264,36 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using _Impl::max_size;
>  
>        // element access
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](const key_type& __x)
>        { return try_emplace(__x).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        operator[](key_type&& __x)
>        { return try_emplace(std::move(__x)).first->second; }
>  
>        template<typename _Key2>
>  	requires __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	operator[](_Key2&& __x)
>  	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>  
> +      _GLIBCXX26_CONSTEXPR
>        mapped_type&
>        at(const key_type& __x)
>        { return at<key_type>(__x); }
>  
> +      _GLIBCXX26_CONSTEXPR
>        const mapped_type&
>        at(const key_type& __x) const
>        { return at<key_type>(__x); }
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	mapped_type&
>  	at(const _Key2& __x)
>  	{
> @@ -1181,6 +1305,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename _Key2>
>  	requires same_as<_Key2, _Key> || __transparent_comparator<_Compare>
> +	_GLIBCXX26_CONSTEXPR
>  	const mapped_type&
>  	at(const _Key2& __x) const
>  	{
> @@ -1203,12 +1328,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(nullopt, __k, std::forward<_Args>(__args)...); }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(key_type&& __k, _Args&&... __args)
>  	{
> @@ -1222,6 +1349,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<mapped_type, _Args...>
>  	  && (!is_convertible_v<_Key2&&, const_iterator>)
>  	  && (!is_convertible_v<_Key2&&, iterator>)
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	try_emplace(_Key2&& __k, _Args&&... __args)
>  	{
> @@ -1231,12 +1359,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, const key_type& __k, _Args&&... __args)
>  	{ return _Impl::_M_try_emplace(__hint, __k, std::forward<_Args>(__args)...).first; }
>  
>        template<typename... _Args>
>  	requires is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
>  	{
> @@ -1248,6 +1378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	requires __transparent_comparator<_Compare>
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_constructible_v<mapped_type, _Args...>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	try_emplace(const_iterator __hint, _Key2&& __k, _Args&&... __args)
>  	{
> @@ -1258,6 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(const key_type& __k, _Mapped&& __obj)
>  	{ return insert_or_assign<const key_type&, _Mapped>(__k, std::forward<_Mapped>(__obj)); }
> @@ -1265,6 +1397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1277,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	pair<iterator, bool>
>  	insert_or_assign(_Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1290,6 +1424,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, const key_type& __k, _Mapped&& __obj)
>  	{
> @@ -1300,6 +1435,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Mapped>
>  	requires is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, key_type&& __k, _Mapped&& __obj)
>  	{
> @@ -1312,6 +1448,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  && is_constructible_v<key_type, _Key2>
>  	  && is_assignable_v<mapped_type&, _Mapped>
>  	  && is_constructible_v<mapped_type, _Mapped>
> +	_GLIBCXX26_CONSTEXPR
>  	iterator
>  	insert_or_assign(const_iterator __hint, _Key2&& __k, _Mapped&& __obj)
>  	{
> @@ -1422,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_map<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> @@ -1590,6 +1728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>    template<typename _Key, typename _Tp, typename _Compare,
>  	   typename _KeyContainer, typename _MappedContainer, typename _Predicate>
> +    _GLIBCXX26_CONSTEXPR
>      typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
>      erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __c,
>  	     _Predicate __pred)
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> index 5bcda3464f1..04c2101aab1 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
> @@ -17,7 +17,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -76,6 +76,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -127,6 +128,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(20,42) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -158,6 +160,7 @@ test02()
>    VERIFY( m.count(3) == 1 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -186,6 +189,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -218,6 +222,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -227,6 +232,7 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, -4, -5}) );
>  }
>  
> +constexpr
>  void
>  test06()
>  {
> @@ -243,6 +249,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -254,6 +261,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
>  }
>  
> +constexpr
>  void
>  test08()
>  {
> @@ -263,6 +271,7 @@ test08()
>    m[k] = 0;
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -274,8 +283,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -290,3 +299,31 @@ main()
>    test08();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test08();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> index f143f3acd1b..00644c14061 100644
> --- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> +++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
> @@ -11,7 +11,7 @@
>  
>  struct Gt {
>    template<typename T, typename U>
> -  bool operator()(T const& l, U const & r) const
> +  constexpr bool operator()(T const& l, U const & r) const
>    { return l > r; }
>  };
>  
> @@ -74,6 +74,7 @@ test_deduction_guide()
>  }
>  
>  template<template<typename> class KeyContainer, template<typename> class MappedContainer>
> +constexpr
>  void
>  test01()
>  {
> @@ -111,6 +112,7 @@ test01()
>    VERIFY( m.end()[-1] == std::pair(12,3) );
>  }
>  
> +constexpr
>  void
>  test02()
>  {
> @@ -137,6 +139,7 @@ test02()
>    VERIFY( m.count(3) == 2 );
>  }
>  
> +constexpr
>  void
>  test03()
>  {
> @@ -165,6 +168,7 @@ test03()
>    VERIFY( std::ranges::equal(m, (std::pair<int, int>[]){{5, 6}}) );
>  }
>  
> +constexpr
>  void
>  test04()
>  {
> @@ -197,6 +201,7 @@ test04()
>    VERIFY( m5.values().get_allocator().get_personality() == 44 );
>  }
>  
> +constexpr
>  void
>  test05()
>  {
> @@ -205,6 +210,8 @@ test05()
>    VERIFY( std::ranges::equal(m | std::views::keys, (int[]){1, 2, 3, 3, 4, 5}) );
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){-1, -2, -3, 3, -4, -5}) );
>  }
> +
> +constexpr
>  void
>  test06()
>  {
> @@ -221,6 +228,7 @@ test06()
>    VERIFY( std::ranges::equal(m | std::views::values, (int[]){2, 3, 4, 5, 6}) );
>  }
>  
> +constexpr
>  void
>  test07()
>  {
> @@ -232,6 +240,7 @@ test07()
>    VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
>  }
>  
> +constexpr
>  void
>  test09()
>  {
> @@ -243,8 +252,8 @@ test09()
>    using value_type = std::pair<int, int>;
>  }
>  
> -int
> -main()
> +void
> +test()
>  {
>    test01<std::vector, std::vector>();
>    test01<std::deque, std::deque>();
> @@ -258,3 +267,30 @@ main()
>    test07();
>    test09();
>  }
> +
> +constexpr
> +bool
> +test_constexpr()
> +{
> +  test01<std::vector, std::vector>();
> +  test02();
> +  test03();
> +  test04();
> +  test05();
> +  test06();
> +  test07();
> +  test09();
> +  return true;
> +}
> +
> +int
> +main()
> +{
> +  test();
> +#if __cplusplus > 202302L
> +  static_assert(test_constexpr());
> +#if __cpp_lib_constexpr_flat_map != 202502L
> +#error "Feature-test macro __cpp_lib_constexpr_flat_map has wrong value in <flat_map>"
> +#endif
> +#endif
> +}
> -- 
> 2.52.0
> 



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