[committed 2/2] libstdc++: Implement constexpr std::basic_string for C++20

Jonathan Wakely jwakely@redhat.com
Tue Nov 16 16:45:27 GMT 2021


From: Michael de Lang <kingoipo@gmail.com>

Tested x86_64-linux, committed to trunk.


This is only supported for the cxx11 ABI, not for COW strings.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string, operator""s): Add
	constexpr for C++20.
	(basic_string::basic_string(basic_string&&)): Only copy
	initialized portion of the buffer.
	(basic_string::basic_string(basic_string&&, const Alloc&)):
	Likewise.
	* include/bits/basic_string.tcc (basic_string): Add constexpr
	for C++20.
	(basic_string::swap(basic_string&)): Only copy initialized
	portions of the buffers.
	(basic_string::_M_replace): Add constexpr implementation that
	doesn't depend on pointer comparisons.
	* include/bits/cow_string.h: Adjust comment.
	* include/ext/type_traits.h (__is_null_pointer): Add constexpr.
	* include/std/string (erase, erase_if): Add constexpr.
	* include/std/version (__cpp_lib_constexpr_string): Update
	value.
	* testsuite/21_strings/basic_string/cons/char/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/literals/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc: New test.
	* testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/version.cc: New test.
---
 libstdc++-v3/include/bits/basic_string.h      | 274 ++++++++++++++++--
 libstdc++-v3/include/bits/basic_string.tcc    |  69 ++++-
 libstdc++-v3/include/bits/cow_string.h        |   2 +-
 libstdc++-v3/include/ext/type_traits.h        |   4 +-
 libstdc++-v3/include/std/string               |   2 +
 libstdc++-v3/include/std/version              |   6 +-
 .../basic_string/cons/char/constexpr.cc       | 174 +++++++++++
 .../basic_string/cons/wchar_t/constexpr.cc    | 174 +++++++++++
 .../basic_string/literals/constexpr.cc        |  22 ++
 .../basic_string/modifiers/constexpr.cc       |  52 ++++
 .../modifiers/swap/char/constexpr.cc          |  49 ++++
 .../modifiers/swap/wchar_t/constexpr.cc       |  49 ++++
 .../21_strings/basic_string/version.cc        |  25 ++
 13 files changed, 869 insertions(+), 33 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/literals/constexpr.cc
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc
 create mode 100644 libstdc++-v3/testsuite/21_strings/basic_string/version.cc

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index a6575fa9e26..b6945f1cdfb 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -57,12 +57,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 #ifdef __cpp_lib_is_constant_evaluated
-// Support P1032R1 in C++20 (but not P0980R1 yet).
-# define __cpp_lib_constexpr_string 201811L
+// Support P0980R1 in C++20.
+# define __cpp_lib_constexpr_string 201907L
 #elif __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
 // Support P0426R1 changes to char_traits in C++17.
 # define __cpp_lib_constexpr_string 201611L
-#elif __cplusplus > 201703L
 #endif
 
   /**
@@ -131,6 +130,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	  _Res>;
 
       // Allows an implicit conversion to __sv_type.
+      _GLIBCXX20_CONSTEXPR
       static __sv_type
       _S_to_string_view(__sv_type __svt) noexcept
       { return __svt; }
@@ -141,7 +141,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // is provided.
       struct __sv_wrapper
       {
-	explicit __sv_wrapper(__sv_type __sv) noexcept : _M_sv(__sv) { }
+	_GLIBCXX20_CONSTEXPR explicit
+	__sv_wrapper(__sv_type __sv) noexcept : _M_sv(__sv) { }
+
 	__sv_type _M_sv;
       };
 
@@ -151,6 +153,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __svw  string view wrapper.
        *  @param  __a  Allocator to use.
        */
+      _GLIBCXX20_CONSTEXPR
       explicit
       basic_string(__sv_wrapper __svw, const _Alloc& __a)
       : basic_string(__svw._M_sv.data(), __svw._M_sv.size(), __a) { }
@@ -163,9 +166,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	_Alloc_hider(pointer __dat, const _Alloc& __a = _Alloc())
 	: allocator_type(__a), _M_p(__dat) { }
 #else
+	_GLIBCXX20_CONSTEXPR
 	_Alloc_hider(pointer __dat, const _Alloc& __a)
 	: allocator_type(__a), _M_p(__dat) { }
 
+	_GLIBCXX20_CONSTEXPR
 	_Alloc_hider(pointer __dat, _Alloc&& __a = _Alloc())
 	: allocator_type(std::move(__a)), _M_p(__dat) { }
 #endif
@@ -184,18 +189,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	size_type        _M_allocated_capacity;
       };
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_data(pointer __p)
       { _M_dataplus._M_p = __p; }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_length(size_type __length)
       { _M_string_length = __length; }
 
+      _GLIBCXX20_CONSTEXPR
       pointer
       _M_data() const
       { return _M_dataplus._M_p; }
 
+      _GLIBCXX20_CONSTEXPR
       pointer
       _M_local_data()
       {
@@ -206,6 +215,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #endif
       }
 
+      _GLIBCXX20_CONSTEXPR
       const_pointer
       _M_local_data() const
       {
@@ -216,10 +226,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #endif
       }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_capacity(size_type __capacity)
       { _M_allocated_capacity = __capacity; }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_set_length(size_type __n)
       {
@@ -227,14 +239,17 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	traits_type::assign(_M_data()[__n], _CharT());
       }
 
+      _GLIBCXX20_CONSTEXPR
       bool
       _M_is_local() const
       { return _M_data() == _M_local_data(); }
 
       // Create & Destroy
+      _GLIBCXX20_CONSTEXPR
       pointer
       _M_create(size_type&, size_type);
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_dispose()
       {
@@ -242,6 +257,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	  _M_destroy(_M_allocated_capacity);
       }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_destroy(size_type __size) throw()
       { _Alloc_traits::deallocate(_M_get_allocator(), _M_data(), __size + 1); }
@@ -249,6 +265,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // _M_construct_aux is used to implement the 21.3.1 para 15 which
       // requires special behaviour if _InIterator is an integral type
       template<typename _InIterator>
+	_GLIBCXX20_CONSTEXPR
         void
         _M_construct_aux(_InIterator __beg, _InIterator __end,
 			 std::__false_type)
@@ -260,15 +277,18 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 438. Ambiguity in the "do the right thing" clause
       template<typename _Integer>
+	_GLIBCXX20_CONSTEXPR
         void
         _M_construct_aux(_Integer __beg, _Integer __end, std::__true_type)
 	{ _M_construct_aux_2(static_cast<size_type>(__beg), __end); }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_construct_aux_2(size_type __req, _CharT __c)
       { _M_construct(__req, __c); }
 
       template<typename _InIterator>
+	_GLIBCXX20_CONSTEXPR
         void
         _M_construct(_InIterator __beg, _InIterator __end)
 	{
@@ -278,6 +298,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
       // For Input Iterators, used in istreambuf_iterators, etc.
       template<typename _InIterator>
+	_GLIBCXX20_CONSTEXPR
         void
         _M_construct(_InIterator __beg, _InIterator __end,
 		     std::input_iterator_tag);
@@ -285,17 +306,21 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // For forward_iterators up to random_access_iterators, used for
       // string::iterator, _CharT*, etc.
       template<typename _FwdIterator>
+	_GLIBCXX20_CONSTEXPR
         void
         _M_construct(_FwdIterator __beg, _FwdIterator __end,
 		     std::forward_iterator_tag);
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_construct(size_type __req, _CharT __c);
 
+      _GLIBCXX20_CONSTEXPR
       allocator_type&
       _M_get_allocator()
       { return _M_dataplus; }
 
+      _GLIBCXX20_CONSTEXPR
       const allocator_type&
       _M_get_allocator() const
       { return _M_dataplus; }
@@ -316,6 +341,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	struct __enable_if_not_native_iterator<_Tp, false> { };
 #endif
 
+      _GLIBCXX20_CONSTEXPR
       size_type
       _M_check(size_type __pos, const char* __s) const
       {
@@ -326,6 +352,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	return __pos;
       }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_check_length(size_type __n1, size_type __n2, const char* __s) const
       {
@@ -335,6 +362,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 
       // NB: _M_limit doesn't check for a bad __pos value.
+      _GLIBCXX20_CONSTEXPR
       size_type
       _M_limit(size_type __pos, size_type __off) const _GLIBCXX_NOEXCEPT
       {
@@ -352,6 +380,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
       // When __n = 1 way faster than the general multichar
       // traits_type::copy/move/assign.
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy(_CharT* __d, const _CharT* __s, size_type __n)
       {
@@ -361,6 +390,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	  traits_type::copy(__d, __s, __n);
       }
 
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_move(_CharT* __d, const _CharT* __s, size_type __n)
       {
@@ -370,6 +400,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	  traits_type::move(__d, __s, __n);
       }
 
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_assign(_CharT* __d, size_type __n, _CharT __c)
       {
@@ -382,6 +413,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // _S_copy_chars is a separate template to permit specialization
       // to optimize for the common case of pointers as iterators.
       template<class _Iterator>
+	_GLIBCXX20_CONSTEXPR
         static void
         _S_copy_chars(_CharT* __p, _Iterator __k1, _Iterator __k2)
         {
@@ -389,24 +421,29 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	    traits_type::assign(*__p, *__k1); // These types are off.
 	}
 
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy_chars(_CharT* __p, iterator __k1, iterator __k2) _GLIBCXX_NOEXCEPT
       { _S_copy_chars(__p, __k1.base(), __k2.base()); }
 
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy_chars(_CharT* __p, const_iterator __k1, const_iterator __k2)
       _GLIBCXX_NOEXCEPT
       { _S_copy_chars(__p, __k1.base(), __k2.base()); }
 
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy_chars(_CharT* __p, _CharT* __k1, _CharT* __k2) _GLIBCXX_NOEXCEPT
       { _S_copy(__p, __k1, __k2 - __k1); }
 
+      _GLIBCXX20_CONSTEXPR
       static void
       _S_copy_chars(_CharT* __p, const _CharT* __k1, const _CharT* __k2)
       _GLIBCXX_NOEXCEPT
       { _S_copy(__p, __k1, __k2 - __k1); }
 
+      _GLIBCXX20_CONSTEXPR
       static int
       _S_compare(size_type __n1, size_type __n2) _GLIBCXX_NOEXCEPT
       {
@@ -420,13 +457,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	  return int(__d);
       }
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_assign(const basic_string&);
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_mutate(size_type __pos, size_type __len1, const _CharT* __s,
 		size_type __len2);
 
+      _GLIBCXX20_CONSTEXPR
       void
       _M_erase(size_type __pos, size_type __n);
 
@@ -438,6 +478,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       /**
        *  @brief  Default constructor creates an empty string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string()
       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
       : _M_dataplus(_M_local_data())
@@ -446,6 +487,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       /**
        *  @brief  Construct an empty string using allocator @a a.
        */
+      _GLIBCXX20_CONSTEXPR
       explicit
       basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
       : _M_dataplus(_M_local_data(), __a)
@@ -455,6 +497,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Construct string with copy of value of @a __str.
        *  @param  __str  Source string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(const basic_string& __str)
       : _M_dataplus(_M_local_data(),
 		    _Alloc_traits::_S_select_on_copy(__str._M_get_allocator()))
@@ -468,6 +511,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __pos  Index of first character to copy from.
        *  @param  __a  Allocator to use.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(const basic_string& __str, size_type __pos,
 		   const _Alloc& __a = _Alloc())
       : _M_dataplus(_M_local_data(), __a)
@@ -483,6 +527,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __pos  Index of first character to copy from.
        *  @param  __n  Number of characters to copy.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(const basic_string& __str, size_type __pos,
 		   size_type __n)
       : _M_dataplus(_M_local_data())
@@ -499,6 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __n  Number of characters to copy.
        *  @param  __a  Allocator to use.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(const basic_string& __str, size_type __pos,
 		   size_type __n, const _Alloc& __a)
       : _M_dataplus(_M_local_data(), __a)
@@ -517,6 +563,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  NB: @a __s must have at least @a __n characters, '\\0'
        *  has no special meaning.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(const _CharT* __s, size_type __n,
 		   const _Alloc& __a = _Alloc())
       : _M_dataplus(_M_local_data(), __a)
@@ -532,6 +579,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // 3076. basic_string CTAD ambiguity
       template<typename = _RequireAllocator<_Alloc>>
 #endif
+      _GLIBCXX20_CONSTEXPR
       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
       : _M_dataplus(_M_local_data(), __a)
       {
@@ -552,6 +600,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // 3076. basic_string CTAD ambiguity
       template<typename = _RequireAllocator<_Alloc>>
 #endif
+      _GLIBCXX20_CONSTEXPR
       basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
       : _M_dataplus(_M_local_data(), __a)
       { _M_construct(__n, __c); }
@@ -564,13 +613,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  The newly-created string contains the exact contents of @a __str.
        *  @a __str is a valid, but unspecified string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(basic_string&& __str) noexcept
       : _M_dataplus(_M_local_data(), std::move(__str._M_get_allocator()))
       {
 	if (__str._M_is_local())
 	  {
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
-			      _S_local_capacity + 1);
+			      __str.length() + 1);
 	  }
 	else
 	  {
@@ -591,14 +641,17 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __l  std::initializer_list of characters.
        *  @param  __a  Allocator to use (default is default allocator).
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
       : _M_dataplus(_M_local_data(), __a)
       { _M_construct(__l.begin(), __l.end()); }
 
+      _GLIBCXX20_CONSTEXPR
       basic_string(const basic_string& __str, const _Alloc& __a)
       : _M_dataplus(_M_local_data(), __a)
       { _M_construct(__str.begin(), __str.end()); }
 
+      _GLIBCXX20_CONSTEXPR
       basic_string(basic_string&& __str, const _Alloc& __a)
       noexcept(_Alloc_traits::_S_always_equal())
       : _M_dataplus(_M_local_data(), __a)
@@ -606,7 +659,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	if (__str._M_is_local())
 	  {
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
-			      _S_local_capacity + 1);
+			      __str.length() + 1);
 	    _M_length(__str.length());
 	    __str._M_set_length(0);
 	  }
@@ -639,6 +692,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #else
       template<typename _InputIterator>
 #endif
+	_GLIBCXX20_CONSTEXPR
         basic_string(_InputIterator __beg, _InputIterator __end,
 		     const _Alloc& __a = _Alloc())
 	: _M_dataplus(_M_local_data(), __a)
@@ -653,6 +707,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __a   Allocator to use.
        */
       template<typename _Tp, typename = _If_sv<_Tp, void>>
+	_GLIBCXX20_CONSTEXPR
 	basic_string(const _Tp& __t, size_type __pos, size_type __n,
 		     const _Alloc& __a = _Alloc())
 	: basic_string(_S_to_string_view(__t).substr(__pos, __n), __a) { }
@@ -663,6 +718,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __a  Allocator to use (default is default allocator).
        */
       template<typename _Tp, typename = _If_sv<_Tp, void>>
+	_GLIBCXX20_CONSTEXPR
 	explicit
 	basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())
 	: basic_string(__sv_wrapper(_S_to_string_view(__t)), __a) { }
@@ -671,6 +727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       /**
        *  @brief  Destroy the string instance.
        */
+      _GLIBCXX20_CONSTEXPR
       ~basic_string()
       { _M_dispose(); }
 
@@ -678,6 +735,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Assign the value of @a str to this string.
        *  @param  __str  Source string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator=(const basic_string& __str)
       {
@@ -688,6 +746,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Copy contents of @a s into this string.
        *  @param  __s  Source null-terminated string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator=(const _CharT* __s)
       { return this->assign(__s); }
@@ -699,6 +758,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Assigning to a character makes this string length 1 and
        *  (*this)[0] == @a c.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator=(_CharT __c)
       {
@@ -716,6 +776,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        */
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 2063. Contradictory requirements for string move assignment
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator=(basic_string&& __str)
       noexcept(_Alloc_traits::_S_nothrow_move())
@@ -784,6 +845,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Set value to string constructed from initializer %list.
        *  @param  __l  std::initializer_list.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator=(initializer_list<_CharT> __l)
       {
@@ -798,6 +860,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __svt  An object convertible to string_view.
        */
      template<typename _Tp>
+       _GLIBCXX20_CONSTEXPR
        _If_sv<_Tp, basic_string&>
        operator=(const _Tp& __svt)
        { return this->assign(__svt); }
@@ -806,6 +869,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Convert to a string_view.
        *  @return A string_view.
        */
+      _GLIBCXX20_CONSTEXPR
       operator __sv_type() const noexcept
       { return __sv_type(data(), size()); }
 #endif // C++17
@@ -815,6 +879,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read/write iterator that points to the first character in
        *  the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       iterator
       begin() _GLIBCXX_NOEXCEPT
       { return iterator(_M_data()); }
@@ -823,6 +888,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read-only (constant) iterator that points to the first
        *  character in the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       const_iterator
       begin() const _GLIBCXX_NOEXCEPT
       { return const_iterator(_M_data()); }
@@ -831,6 +897,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read/write iterator that points one past the last
        *  character in the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       iterator
       end() _GLIBCXX_NOEXCEPT
       { return iterator(_M_data() + this->size()); }
@@ -839,6 +906,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read-only (constant) iterator that points one past the
        *  last character in the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       const_iterator
       end() const _GLIBCXX_NOEXCEPT
       { return const_iterator(_M_data() + this->size()); }
@@ -848,6 +916,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  character in the %string.  Iteration is done in reverse element
        *  order.
        */
+      _GLIBCXX20_CONSTEXPR
       reverse_iterator
       rbegin() _GLIBCXX_NOEXCEPT
       { return reverse_iterator(this->end()); }
@@ -857,6 +926,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  to the last character in the %string.  Iteration is done in
        *  reverse element order.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reverse_iterator
       rbegin() const _GLIBCXX_NOEXCEPT
       { return const_reverse_iterator(this->end()); }
@@ -866,6 +936,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  first character in the %string.  Iteration is done in reverse
        *  element order.
        */
+      _GLIBCXX20_CONSTEXPR
       reverse_iterator
       rend() _GLIBCXX_NOEXCEPT
       { return reverse_iterator(this->begin()); }
@@ -875,6 +946,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  to one before the first character in the %string.  Iteration
        *  is done in reverse element order.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reverse_iterator
       rend() const _GLIBCXX_NOEXCEPT
       { return const_reverse_iterator(this->begin()); }
@@ -884,6 +956,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read-only (constant) iterator that points to the first
        *  character in the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       const_iterator
       cbegin() const noexcept
       { return const_iterator(this->_M_data()); }
@@ -892,6 +965,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read-only (constant) iterator that points one past the
        *  last character in the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       const_iterator
       cend() const noexcept
       { return const_iterator(this->_M_data() + this->size()); }
@@ -901,6 +975,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  to the last character in the %string.  Iteration is done in
        *  reverse element order.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reverse_iterator
       crbegin() const noexcept
       { return const_reverse_iterator(this->end()); }
@@ -910,6 +985,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  to one before the first character in the %string.  Iteration
        *  is done in reverse element order.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reverse_iterator
       crend() const noexcept
       { return const_reverse_iterator(this->begin()); }
@@ -919,17 +995,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // Capacity:
       ///  Returns the number of characters in the string, not including any
       ///  null-termination.
+      _GLIBCXX20_CONSTEXPR
       size_type
       size() const _GLIBCXX_NOEXCEPT
       { return _M_string_length; }
 
       ///  Returns the number of characters in the string, not including any
       ///  null-termination.
+      _GLIBCXX20_CONSTEXPR
       size_type
       length() const _GLIBCXX_NOEXCEPT
       { return _M_string_length; }
 
       ///  Returns the size() of the largest possible %string.
+      _GLIBCXX20_CONSTEXPR
       size_type
       max_size() const _GLIBCXX_NOEXCEPT
       { return (_Alloc_traits::max_size(_M_get_allocator()) - 1) / 2; }
@@ -944,6 +1023,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  %string's current size the %string is truncated, otherwise
        *  the %string is extended and new elements are %set to @a __c.
        */
+      _GLIBCXX20_CONSTEXPR
       void
       resize(size_type __n, _CharT __c);
 
@@ -957,6 +1037,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  are default-constructed.  For basic types such as char, this means
        *  setting them to 0.
        */
+      _GLIBCXX20_CONSTEXPR
       void
       resize(size_type __n)
       { this->resize(__n, _CharT()); }
@@ -965,6 +1046,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
       ///  A non-binding request to reduce capacity() to size().
+      _GLIBCXX20_CONSTEXPR
       void
       shrink_to_fit() noexcept
       { reserve(); }
@@ -982,6 +1064,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns the total number of characters that the %string can hold
        *  before needing to allocate more memory.
        */
+      _GLIBCXX20_CONSTEXPR
       size_type
       capacity() const _GLIBCXX_NOEXCEPT
       {
@@ -1006,6 +1089,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  prevent a possible reallocation of memory and copying of %string
        *  data.
        */
+      _GLIBCXX20_CONSTEXPR
       void
       reserve(size_type __res_arg);
 
@@ -1015,12 +1099,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #if __cplusplus > 201703L
       [[deprecated("use shrink_to_fit() instead")]]
 #endif
+      _GLIBCXX20_CONSTEXPR
       void
       reserve();
 
       /**
        *  Erases the string, making it empty.
        */
+      _GLIBCXX20_CONSTEXPR
       void
       clear() _GLIBCXX_NOEXCEPT
       { _M_set_length(0); }
@@ -1029,7 +1115,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns true if the %string is empty.  Equivalent to 
        *  <code>*this == ""</code>.
        */
-      _GLIBCXX_NODISCARD bool
+      _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
+      bool
       empty() const _GLIBCXX_NOEXCEPT
       { return this->size() == 0; }
 
@@ -1044,6 +1131,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  out_of_range lookups are not defined. (For checked lookups
        *  see at().)
        */
+      _GLIBCXX20_CONSTEXPR
       const_reference
       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
       {
@@ -1061,6 +1149,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  out_of_range lookups are not defined. (For checked lookups
        *  see at().)
        */
+      _GLIBCXX20_CONSTEXPR
       reference
       operator[](size_type __pos)
       {
@@ -1082,6 +1171,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  first checked that it is in the range of the string.  The function
        *  throws out_of_range if the check fails.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reference
       at(size_type __n) const
       {
@@ -1103,6 +1193,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  first checked that it is in the range of the string.  The function
        *  throws out_of_range if the check fails.
        */
+      _GLIBCXX20_CONSTEXPR
       reference
       at(size_type __n)
       {
@@ -1119,6 +1210,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read/write reference to the data at the first
        *  element of the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       reference
       front() noexcept
       {
@@ -1130,6 +1222,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read-only (constant) reference to the data at the first
        *  element of the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reference
       front() const noexcept
       {
@@ -1141,6 +1234,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read/write reference to the data at the last
        *  element of the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       reference
       back() noexcept
       {
@@ -1152,6 +1246,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Returns a read-only (constant) reference to the data at the
        *  last element of the %string.
        */
+      _GLIBCXX20_CONSTEXPR
       const_reference
       back() const noexcept
       {
@@ -1166,6 +1261,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __str  The string to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator+=(const basic_string& __str)
       { return this->append(__str); }
@@ -1175,6 +1271,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __s  The C string to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator+=(const _CharT* __s)
       { return this->append(__s); }
@@ -1184,6 +1281,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __c  The character to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator+=(_CharT __c)
       {
@@ -1197,6 +1295,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __l  The initializer_list of characters to be appended.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       operator+=(initializer_list<_CharT> __l)
       { return this->append(__l.begin(), __l.size()); }
@@ -1209,6 +1308,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	operator+=(const _Tp& __svt)
 	{ return this->append(__svt); }
@@ -1219,6 +1319,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __str  The string to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       append(const basic_string& __str)
       { return _M_append(__str._M_data(), __str.size()); }
@@ -1236,6 +1337,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  than the number of available characters in @a __str, the
        *  remainder of @a __str is appended.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       append(const basic_string& __str, size_type __pos, size_type __n = npos)
       { return _M_append(__str._M_data()
@@ -1248,6 +1350,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __n  The number of characters to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       append(const _CharT* __s, size_type __n)
       {
@@ -1261,6 +1364,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __s  The C string to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       append(const _CharT* __s)
       {
@@ -1278,6 +1382,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *
        *  Appends __n copies of __c to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       append(size_type __n, _CharT __c)
       { return _M_replace_aux(this->size(), size_type(0), __n, __c); }
@@ -1288,6 +1393,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __l  The initializer_list of characters to append.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       append(initializer_list<_CharT> __l)
       { return this->append(__l.begin(), __l.size()); }
@@ -1304,6 +1410,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #if __cplusplus >= 201103L
       template<class _InputIterator,
 	       typename = std::_RequireInputIter<_InputIterator>>
+	_GLIBCXX20_CONSTEXPR
 #else
       template<class _InputIterator>
 #endif
@@ -1318,6 +1425,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
         _If_sv<_Tp, basic_string&>
         append(const _Tp& __svt)
         {
@@ -1333,6 +1441,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
         _If_sv<_Tp, basic_string&>
 	append(const _Tp& __svt, size_type __pos, size_type __n = npos)
 	{
@@ -1347,6 +1456,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Append a single character.
        *  @param __c  Character to append.
        */
+      _GLIBCXX20_CONSTEXPR
       void
       push_back(_CharT __c)
       {
@@ -1362,6 +1472,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param  __str  Source string to use.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(const basic_string& __str)
       {
@@ -1407,6 +1518,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  This function sets this string to the exact contents of @a __str.
        *  @a __str is a valid, but unspecified string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(basic_string&& __str)
       noexcept(_Alloc_traits::_S_nothrow_move())
@@ -1430,6 +1542,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  is larger than the number of available characters in @a
        *  __str, the remainder of @a __str is used.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(const basic_string& __str, size_type __pos, size_type __n = npos)
       { return _M_replace(size_type(0), this->size(), __str._M_data()
@@ -1446,6 +1559,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  characters of @a __s.  If @a __n is is larger than the number of
        *  available characters in @a __s, the remainder of @a __s is used.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(const _CharT* __s, size_type __n)
       {
@@ -1462,6 +1576,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  The data is copied, so there is no dependence on @a __s once the
        *  function returns.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(const _CharT* __s)
       {
@@ -1479,6 +1594,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  This function sets the value of this string to @a __n copies of
        *  character @a __c.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(size_type __n, _CharT __c)
       { return _M_replace_aux(size_type(0), this->size(), __n, __c); }
@@ -1494,6 +1610,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #if __cplusplus >= 201103L
       template<class _InputIterator,
 	       typename = std::_RequireInputIter<_InputIterator>>
+	_GLIBCXX20_CONSTEXPR
 #else
       template<class _InputIterator>
 #endif
@@ -1507,6 +1624,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __l  The initializer_list of characters to assign.
        *  @return  Reference to this string.
        */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       assign(initializer_list<_CharT> __l)
       { return this->assign(__l.begin(), __l.size()); }
@@ -1519,6 +1637,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	assign(const _Tp& __svt)
 	{
@@ -1534,6 +1653,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	assign(const _Tp& __svt, size_type __pos, size_type __n = npos)
 	{
@@ -1561,6 +1681,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  length_error is thrown.  The value of the string doesn't
        *  change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       iterator
       insert(const_iterator __p, size_type __n, _CharT __c)
       {
@@ -1605,6 +1726,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       */
       template<class _InputIterator,
 	       typename = std::_RequireInputIter<_InputIterator>>
+	_GLIBCXX20_CONSTEXPR
 	iterator
         insert(const_iterator __p, _InputIterator __beg, _InputIterator __end)
         {
@@ -1639,6 +1761,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __l  The initializer_list of characters to insert.
        *  @throw  std::length_error  If new length exceeds @c max_size().
        */
+      _GLIBCXX20_CONSTEXPR
       iterator
       insert(const_iterator __p, initializer_list<_CharT> __l)
       { return this->insert(__p, __l.begin(), __l.end()); }
@@ -1666,6 +1789,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  length_error is thrown.  The value of the string doesn't
        *  change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       insert(size_type __pos1, const basic_string& __str)
       { return this->replace(__pos1, size_type(0),
@@ -1689,6 +1813,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  beyond the end of @a __str, out_of_range is thrown.  The
        *  value of the string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       insert(size_type __pos1, const basic_string& __str,
 	     size_type __pos2, size_type __n = npos)
@@ -1712,6 +1837,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  end(), out_of_range is thrown.  The value of the string
        *  doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       insert(size_type __pos, const _CharT* __s, size_type __n)
       { return this->replace(__pos, size_type(0), __s, __n); }
@@ -1731,6 +1857,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  thrown.  The value of the string doesn't change if an error is
        *  thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       insert(size_type __pos, const _CharT* __s)
       {
@@ -1755,6 +1882,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  out_of_range is thrown.  The value of the string doesn't
        *  change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       insert(size_type __pos, size_type __n, _CharT __c)
       { return _M_replace_aux(_M_check(__pos, "basic_string::insert"),
@@ -1773,6 +1901,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  out_of_range is thrown.  The value of the string doesn't
        *  change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       iterator
       insert(__const_iterator __p, _CharT __c)
       {
@@ -1790,6 +1919,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	insert(size_type __pos, const _Tp& __svt)
 	{
@@ -1806,6 +1936,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	insert(size_type __pos1, const _Tp& __svt,
 	       size_type __pos2, size_type __n = npos)
@@ -1833,6 +1964,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  out_of_range is thrown.  The value of the string doesn't
        *  change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       erase(size_type __pos = 0, size_type __n = npos)
       {
@@ -1852,6 +1984,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Removes the character at @a __position from this string. The value
        *  of the string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       iterator
       erase(__const_iterator __position)
       {
@@ -1871,6 +2004,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Removes the characters in the range [first,last) from this string.
        *  The value of the string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       iterator
       erase(__const_iterator __first, __const_iterator __last)
       {
@@ -1890,6 +2024,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *
        *  The string must be non-empty.
        */
+      _GLIBCXX20_CONSTEXPR
       void
       pop_back() noexcept
       {
@@ -1915,6 +2050,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  is thrown.  The value of the string doesn't change if an
        *  error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(size_type __pos, size_type __n, const basic_string& __str)
       { return this->replace(__pos, __n, __str._M_data(), __str.size()); }
@@ -1937,6 +2073,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  result exceeds max_size(), length_error is thrown.  The value of the
        *  string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(size_type __pos1, size_type __n1, const basic_string& __str,
 	      size_type __pos2, size_type __n2 = npos)
@@ -1962,6 +2099,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  thrown.  The value of the string doesn't change if an error
        *  is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(size_type __pos, size_type __n1, const _CharT* __s,
 	      size_type __n2)
@@ -1987,6 +2125,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  length_error is thrown.  The value of the string doesn't
        *  change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(size_type __pos, size_type __n1, const _CharT* __s)
       {
@@ -2011,6 +2150,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  thrown.  The value of the string doesn't change if an error
        *  is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(size_type __pos, size_type __n1, size_type __n2, _CharT __c)
       { return _M_replace_aux(_M_check(__pos, "basic_string::replace"),
@@ -2029,6 +2169,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  exceeds max_size(), length_error is thrown.  The value of
        *  the string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2,
 	      const basic_string& __str)
@@ -2049,6 +2190,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  The value of the string doesn't change if an error is
        *  thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2,
 	      const _CharT* __s, size_type __n)
@@ -2071,6 +2213,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  result exceeds max_size(), length_error is thrown.  The
        *  value of the string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2, const _CharT* __s)
       {
@@ -2092,6 +2235,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  result exceeds max_size(), length_error is thrown.  The
        *  value of the string doesn't change if an error is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2, size_type __n,
 	      _CharT __c)
@@ -2119,6 +2263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #if __cplusplus >= 201103L
       template<class _InputIterator,
 	       typename = std::_RequireInputIter<_InputIterator>>
+	_GLIBCXX20_CONSTEXPR
         basic_string&
         replace(const_iterator __i1, const_iterator __i2,
 		_InputIterator __k1, _InputIterator __k2)
@@ -2149,6 +2294,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
       // Specializations for the common case of pointer and iterator:
       // useful to avoid the overhead of temporary buffering in _M_replace.
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2,
 	      _CharT* __k1, _CharT* __k2)
@@ -2160,6 +2306,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 			     __k1, __k2 - __k1);
       }
 
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2,
 	      const _CharT* __k1, const _CharT* __k2)
@@ -2171,6 +2318,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 			     __k1, __k2 - __k1);
       }
 
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2,
 	      iterator __k1, iterator __k2)
@@ -2182,6 +2330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 			     __k1.base(), __k2 - __k1);
       }
 
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       replace(__const_iterator __i1, __const_iterator __i2,
 	      const_iterator __k1, const_iterator __k2)
@@ -2208,6 +2357,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  The value of the string doesn't change if an error is
        *  thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string& replace(const_iterator __i1, const_iterator __i2,
 			    initializer_list<_CharT> __l)
       { return this->replace(__i1, __i2, __l.begin(), __l.size()); }
@@ -2222,6 +2372,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	replace(size_type __pos, size_type __n, const _Tp& __svt)
 	{
@@ -2239,6 +2390,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	replace(size_type __pos1, size_type __n1, const _Tp& __svt,
 		size_type __pos2, size_type __n2 = npos)
@@ -2260,6 +2412,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Reference to this string.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, basic_string&>
 	replace(const_iterator __i1, const_iterator __i2, const _Tp& __svt)
 	{
@@ -2270,25 +2423,30 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     private:
       template<class _Integer>
+	_GLIBCXX20_CONSTEXPR
 	basic_string&
 	_M_replace_dispatch(const_iterator __i1, const_iterator __i2,
 			    _Integer __n, _Integer __val, __true_type)
         { return _M_replace_aux(__i1 - begin(), __i2 - __i1, __n, __val); }
 
       template<class _InputIterator>
+	_GLIBCXX20_CONSTEXPR
 	basic_string&
 	_M_replace_dispatch(const_iterator __i1, const_iterator __i2,
 			    _InputIterator __k1, _InputIterator __k2,
 			    __false_type);
 
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       _M_replace_aux(size_type __pos1, size_type __n1, size_type __n2,
 		     _CharT __c);
 
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       _M_replace(size_type __pos, size_type __len1, const _CharT* __s,
 		 const size_type __len2);
 
+      _GLIBCXX20_CONSTEXPR
       basic_string&
       _M_append(const _CharT* __s, size_type __n);
 
@@ -2306,6 +2464,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  C string @a __s.  If @a __pos is %greater than size(),
        *  out_of_range is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       copy(_CharT* __s, size_type __n, size_type __pos = 0) const;
 
@@ -2316,6 +2475,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  Exchanges the contents of this string with that of @a __s in constant
        *  time.
       */
+      _GLIBCXX20_CONSTEXPR
       void
       swap(basic_string& __s) _GLIBCXX_NOEXCEPT;
 
@@ -2326,6 +2486,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  This is a handle to internal data.  Do not modify or dire things may
        *  happen.
       */
+      _GLIBCXX20_CONSTEXPR
       const _CharT*
       c_str() const _GLIBCXX_NOEXCEPT
       { return _M_data(); }
@@ -2338,6 +2499,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  allows modifying the contents use @c &str[0] instead,
        *  (or in C++17 the non-const @c str.data() overload).
       */
+      _GLIBCXX20_CONSTEXPR
       const _CharT*
       data() const _GLIBCXX_NOEXCEPT
       { return _M_data(); }
@@ -2349,6 +2511,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  This is a pointer to the character sequence held by the string.
        *  Modifying the characters in the sequence is allowed.
       */
+      _GLIBCXX20_CONSTEXPR
       _CharT*
       data() noexcept
       { return _M_data(); }
@@ -2357,6 +2520,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       /**
        *  @brief  Return copy of allocator used to construct this string.
       */
+      _GLIBCXX20_CONSTEXPR
       allocator_type
       get_allocator() const _GLIBCXX_NOEXCEPT
       { return _M_get_allocator(); }
@@ -2373,6 +2537,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  returns the index where it begins.  If not found, returns
        *  npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find(const _CharT* __s, size_type __pos, size_type __n) const
       _GLIBCXX_NOEXCEPT;
@@ -2387,6 +2552,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  this string.  If found, returns the index where it begins.  If not
        *  found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find(const basic_string& __str, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT
@@ -2400,6 +2566,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of start of first occurrence.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, size_type>
 	find(const _Tp& __svt, size_type __pos = 0) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2419,6 +2586,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  __s within this string.  If found, returns the index where
        *  it begins.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find(const _CharT* __s, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
       {
@@ -2436,6 +2604,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  this string.  If found, returns the index where it was
        *  found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT;
 
@@ -2449,6 +2618,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  __str within this string.  If found, returns the index where
        *  it begins.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       rfind(const basic_string& __str, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT
@@ -2462,6 +2632,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of start of last occurrence.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, size_type>
 	rfind(const _Tp& __svt, size_type __pos = npos) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2483,6 +2654,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  returns the index where it begins.  If not found, returns
        *  npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       rfind(const _CharT* __s, size_type __pos, size_type __n) const
       _GLIBCXX_NOEXCEPT;
@@ -2497,6 +2669,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @a __s within this string.  If found, returns the index
        *  where it begins.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       rfind(const _CharT* __s, size_type __pos = npos) const
       {
@@ -2514,6 +2687,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  this string.  If found, returns the index where it was
        *  found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       rfind(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT;
 
@@ -2528,6 +2702,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  returns the index where it was found.  If not found, returns
        *  npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_of(const basic_string& __str, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT
@@ -2542,6 +2717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of first occurrence.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, size_type>
 	find_first_of(const _Tp& __svt, size_type __pos = 0) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2563,6 +2739,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  found, returns the index where it was found.  If not found,
        *  returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_of(const _CharT* __s, size_type __pos, size_type __n) const
       _GLIBCXX_NOEXCEPT;
@@ -2577,6 +2754,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  characters of @a __s within this string.  If found, returns
        *  the index where it was found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_of(const _CharT* __s, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT
@@ -2597,6 +2775,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *
        *  Note: equivalent to find(__c, __pos).
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_of(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
       { return this->find(__c, __pos); }
@@ -2612,6 +2791,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  returns the index where it was found.  If not found, returns
        *  npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_of(const basic_string& __str, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT
@@ -2626,6 +2806,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of last occurrence.
       */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, size_type>
 	find_last_of(const _Tp& __svt, size_type __pos = npos) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2647,6 +2828,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  found, returns the index where it was found.  If not found,
        *  returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_of(const _CharT* __s, size_type __pos, size_type __n) const
       _GLIBCXX_NOEXCEPT;
@@ -2661,6 +2843,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  characters of @a __s within this string.  If found, returns
        *  the index where it was found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_of(const _CharT* __s, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT
@@ -2681,6 +2864,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *
        *  Note: equivalent to rfind(__c, __pos).
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_of(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT
       { return this->rfind(__c, __pos); }
@@ -2695,6 +2879,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  in @a __str within this string.  If found, returns the index where it
        *  was found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_not_of(const basic_string& __str, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT
@@ -2710,6 +2895,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        */
       template<typename _Tp>
 	_If_sv<_Tp, size_type>
+	_GLIBCXX20_CONSTEXPR
 	find_first_not_of(const _Tp& __svt, size_type __pos = 0) const
 	noexcept(is_same<_Tp, __sv_type>::value)
 	{
@@ -2730,6 +2916,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  this string.  If found, returns the index where it was
        *  found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_not_of(const _CharT* __s, size_type __pos,
 			size_type __n) const _GLIBCXX_NOEXCEPT;
@@ -2744,6 +2931,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  contained in @a __s within this string.  If found, returns
        *  the index where it was found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_not_of(const _CharT* __s, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT
@@ -2762,6 +2950,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  other than @a __c within this string.  If found, returns the
        *  index where it was found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_first_not_of(_CharT __c, size_type __pos = 0) const
       _GLIBCXX_NOEXCEPT;
@@ -2777,6 +2966,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  returns the index where it was found.  If not found, returns
        *  npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_not_of(const basic_string& __str, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT
@@ -2791,6 +2981,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of last occurrence.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, size_type>
 	find_last_not_of(const _Tp& __svt, size_type __pos = npos) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2812,6 +3003,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  If found, returns the index where it was found.  If not found,
        *  returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_not_of(const _CharT* __s, size_type __pos,
 		       size_type __n) const _GLIBCXX_NOEXCEPT;
@@ -2826,6 +3018,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  returns the index where it was found.  If not found, returns
        *  npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_not_of(const _CharT* __s, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT
@@ -2844,6 +3037,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @a __c within this string.  If found, returns the index where it was
        *  found.  If not found, returns npos.
       */
+      _GLIBCXX20_CONSTEXPR
       size_type
       find_last_not_of(_CharT __c, size_type __pos = npos) const
       _GLIBCXX_NOEXCEPT;
@@ -2860,6 +3054,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  short, use the remainder of the characters.  If @a __pos is
        *  beyond the end of the string, out_of_range is thrown.
       */
+      _GLIBCXX20_CONSTEXPR
       basic_string
       substr(size_type __pos = 0, size_type __n = npos) const
       { return basic_string(*this,
@@ -2879,6 +3074,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  If the result of the comparison is nonzero returns it,
        *  otherwise the shorter one is ordered first.
       */
+      _GLIBCXX20_CONSTEXPR
       int
       compare(const basic_string& __str) const
       {
@@ -2899,6 +3095,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Integer < 0, 0, or > 0.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, int>
 	compare(const _Tp& __svt) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2923,6 +3120,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Integer < 0, 0, or > 0.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, int>
 	compare(size_type __pos, size_type __n, const _Tp& __svt) const
 	noexcept(is_same<_Tp, __sv_type>::value)
@@ -2942,6 +3140,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Integer < 0, 0, or > 0.
        */
       template<typename _Tp>
+	_GLIBCXX20_CONSTEXPR
 	_If_sv<_Tp, int>
 	compare(size_type __pos1, size_type __n1, const _Tp& __svt,
 		size_type __pos2, size_type __n2 = npos) const
@@ -2972,6 +3171,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  result of the comparison is nonzero returns it, otherwise
        *  the shorter one is ordered first.
       */
+      _GLIBCXX20_CONSTEXPR
       int
       compare(size_type __pos, size_type __n, const basic_string& __str) const;
 
@@ -2998,6 +3198,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  If the result of the comparison is nonzero returns it,
        *  otherwise the shorter one is ordered first.
       */
+      _GLIBCXX20_CONSTEXPR
       int
       compare(size_type __pos1, size_type __n1, const basic_string& __str,
 	      size_type __pos2, size_type __n2 = npos) const;
@@ -3016,6 +3217,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  comparison is nonzero returns it, otherwise the shorter one is
        *  ordered first.
       */
+      _GLIBCXX20_CONSTEXPR
       int
       compare(const _CharT* __s) const _GLIBCXX_NOEXCEPT;
 
@@ -3040,6 +3242,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  the comparison is nonzero returns it, otherwise the shorter
        *  one is ordered first.
       */
+      _GLIBCXX20_CONSTEXPR
       int
       compare(size_type __pos, size_type __n1, const _CharT* __s) const;
 
@@ -3067,46 +3270,47 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  NB: s must have at least n2 characters, '\\0' has
        *  no special meaning.
       */
+      _GLIBCXX20_CONSTEXPR
       int
       compare(size_type __pos, size_type __n1, const _CharT* __s,
 	      size_type __n2) const;
 
-#if __cplusplus > 201703L
-      bool
+#if __cplusplus >= 202002L
+      constexpr bool
       starts_with(basic_string_view<_CharT, _Traits> __x) const noexcept
       { return __sv_type(this->data(), this->size()).starts_with(__x); }
 
-      bool
+      constexpr bool
       starts_with(_CharT __x) const noexcept
       { return __sv_type(this->data(), this->size()).starts_with(__x); }
 
-      bool
+      constexpr bool
       starts_with(const _CharT* __x) const noexcept
       { return __sv_type(this->data(), this->size()).starts_with(__x); }
 
-      bool
+      constexpr bool
       ends_with(basic_string_view<_CharT, _Traits> __x) const noexcept
       { return __sv_type(this->data(), this->size()).ends_with(__x); }
 
-      bool
+      constexpr bool
       ends_with(_CharT __x) const noexcept
       { return __sv_type(this->data(), this->size()).ends_with(__x); }
 
-      bool
+      constexpr bool
       ends_with(const _CharT* __x) const noexcept
       { return __sv_type(this->data(), this->size()).ends_with(__x); }
 #endif // C++20
 
 #if __cplusplus > 202002L
-      bool
+      constexpr bool
       contains(basic_string_view<_CharT, _Traits> __x) const noexcept
       { return __sv_type(this->data(), this->size()).contains(__x); }
 
-      bool
+      constexpr bool
       contains(_CharT __x) const noexcept
       { return __sv_type(this->data(), this->size()).contains(__x); }
 
-      bool
+      constexpr bool
       contains(const _CharT* __x) const noexcept
       { return __sv_type(this->data(), this->size()).contains(__x); }
 #endif // C++23
@@ -3160,6 +3364,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  New string with value of @a __lhs followed by @a __rhs.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT, _Traits, _Alloc>
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 	      const basic_string<_CharT, _Traits, _Alloc>& __rhs)
@@ -3176,6 +3381,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  New string with value of @a __lhs followed by @a __rhs.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT,_Traits,_Alloc>
     operator+(const _CharT* __lhs,
 	      const basic_string<_CharT,_Traits,_Alloc>& __rhs);
@@ -3187,6 +3393,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  New string with @a __lhs followed by @a __rhs.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT,_Traits,_Alloc>
     operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>& __rhs);
 
@@ -3197,6 +3404,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  New string with @a __lhs followed by @a __rhs.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 	      const _CharT* __rhs)
@@ -3213,6 +3421,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  New string with @a __lhs followed by @a __rhs.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, _CharT __rhs)
     {
@@ -3225,18 +3434,21 @@ _GLIBCXX_END_NAMESPACE_CXX11
 
 #if __cplusplus >= 201103L
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
 	      const basic_string<_CharT, _Traits, _Alloc>& __rhs)
     { return std::move(__lhs.append(__rhs)); }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 	      basic_string<_CharT, _Traits, _Alloc>&& __rhs)
     { return std::move(__rhs.insert(0, __lhs)); }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
 	      basic_string<_CharT, _Traits, _Alloc>&& __rhs)
@@ -3259,24 +3471,28 @@ _GLIBCXX_END_NAMESPACE_CXX11
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(const _CharT* __lhs,
 	      basic_string<_CharT, _Traits, _Alloc>&& __rhs)
     { return std::move(__rhs.insert(0, __lhs)); }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(_CharT __lhs,
 	      basic_string<_CharT, _Traits, _Alloc>&& __rhs)
     { return std::move(__rhs.insert(0, 1, __lhs)); }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
 	      const _CharT* __rhs)
     { return std::move(__lhs.append(__rhs)); }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline basic_string<_CharT, _Traits, _Alloc>
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
 	      _CharT __rhs)
@@ -3291,6 +3507,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  True if @a __lhs.compare(@a __rhs) == 0.  False otherwise.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 	       const basic_string<_CharT, _Traits, _Alloc>& __rhs)
@@ -3298,6 +3515,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
     { return __lhs.compare(__rhs) == 0; }
 
   template<typename _CharT>
+    _GLIBCXX20_CONSTEXPR
     inline
     typename __gnu_cxx::__enable_if<__is_char<_CharT>::__value, bool>::__type
     operator==(const basic_string<_CharT>& __lhs,
@@ -3313,6 +3531,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  @return  True if @a __lhs.compare(@a __rhs) == 0.  False otherwise.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 	       const _CharT* __rhs)
@@ -3327,6 +3546,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *	       greater than, or incomparable with `__rhs`.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline auto
     operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 		const basic_string<_CharT, _Traits, _Alloc>& __rhs) noexcept
@@ -3341,6 +3561,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *	       greater than, or incomparable with `__rhs`.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline auto
     operator<=>(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
 		const _CharT* __rhs) noexcept
@@ -3558,6 +3779,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
    *  Exchanges the contents of @a __lhs and @a __rhs in constant time.
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     inline void
     swap(basic_string<_CharT, _Traits, _Alloc>& __lhs,
 	 basic_string<_CharT, _Traits, _Alloc>& __rhs)
@@ -4034,33 +4256,41 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   {
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wliteral-suffix"
-    _GLIBCXX_DEFAULT_ABI_TAG
+
+#if __cpp_lib_constexpr_string >= 201907L
+# define _GLIBCXX_STRING_CONSTEXPR constexpr
+#else
+# define _GLIBCXX_STRING_CONSTEXPR
+#endif
+
+    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
     inline basic_string<char>
     operator""s(const char* __str, size_t __len)
     { return basic_string<char>{__str, __len}; }
 
-    _GLIBCXX_DEFAULT_ABI_TAG
+    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
     inline basic_string<wchar_t>
     operator""s(const wchar_t* __str, size_t __len)
     { return basic_string<wchar_t>{__str, __len}; }
 
 #ifdef _GLIBCXX_USE_CHAR8_T
-    _GLIBCXX_DEFAULT_ABI_TAG
+    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
     inline basic_string<char8_t>
     operator""s(const char8_t* __str, size_t __len)
     { return basic_string<char8_t>{__str, __len}; }
 #endif
 
-    _GLIBCXX_DEFAULT_ABI_TAG
+    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
     inline basic_string<char16_t>
     operator""s(const char16_t* __str, size_t __len)
     { return basic_string<char16_t>{__str, __len}; }
 
-    _GLIBCXX_DEFAULT_ABI_TAG
+    _GLIBCXX_DEFAULT_ABI_TAG _GLIBCXX_STRING_CONSTEXPR
     inline basic_string<char32_t>
     operator""s(const char32_t* __str, size_t __len)
     { return basic_string<char32_t>{__str, __len}; }
 
+#undef _GLIBCXX_STRING_CONSTEXPR
 #pragma GCC diagnostic pop
   } // inline namespace string_literals
   } // inline namespace literals
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 98c386239f9..5743770b42a 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -54,6 +54,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     basic_string<_CharT, _Traits, _Alloc>::npos;
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     swap(basic_string& __s) _GLIBCXX_NOEXCEPT
@@ -70,16 +71,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      {
 		_CharT __tmp_data[_S_local_capacity + 1];
 		traits_type::copy(__tmp_data, __s._M_local_buf,
-				  _S_local_capacity + 1);
+				  __s.length() + 1);
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
-				  _S_local_capacity + 1);
+				  length() + 1);
 		traits_type::copy(_M_local_buf, __tmp_data,
-				  _S_local_capacity + 1);
+				  __s.length() + 1);
 	      }
 	    else if (__s.length())
 	      {
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
-				  _S_local_capacity + 1);
+				  __s.length() + 1);
 		_M_length(__s.length());
 		__s._M_set_length(0);
 		return;
@@ -87,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    else if (length())
 	      {
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
-				  _S_local_capacity + 1);
+				  length() + 1);
 		__s._M_length(length());
 		_M_set_length(0);
 		return;
@@ -97,7 +98,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
-			      _S_local_capacity + 1);
+			      length() + 1);
 	    _M_data(__s._M_data());
 	    __s._M_data(__s._M_local_buf);
 	    _M_capacity(__tmp_capacity);
@@ -108,7 +109,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  if (__s._M_is_local())
 	    {
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
-				_S_local_capacity + 1);
+				__s.length() + 1);
 	      __s._M_data(_M_data());
 	      _M_data(_M_local_buf);
 	    }
@@ -128,6 +129,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::pointer
     basic_string<_CharT, _Traits, _Alloc>::
     _M_create(size_type& __capacity, size_type __old_capacity)
@@ -159,6 +161,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // pointers, calling for a different coding style.
   template<typename _CharT, typename _Traits, typename _Alloc>
     template<typename _InIterator>
+      _GLIBCXX20_CONSTEXPR
       void
       basic_string<_CharT, _Traits, _Alloc>::
       _M_construct(_InIterator __beg, _InIterator __end,
@@ -202,6 +205,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _CharT, typename _Traits, typename _Alloc>
     template<typename _InIterator>
+      _GLIBCXX20_CONSTEXPR
       void
       basic_string<_CharT, _Traits, _Alloc>::
       _M_construct(_InIterator __beg, _InIterator __end,
@@ -233,6 +237,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     _M_construct(size_type __n, _CharT __c)
@@ -250,6 +255,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     _M_assign(const basic_string& __str)
@@ -276,6 +282,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     reserve(size_type __res)
@@ -296,6 +303,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     _M_mutate(size_type __pos, size_type __len1, const _CharT* __s,
@@ -320,6 +328,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     _M_erase(size_type __pos, size_type __n)
@@ -333,6 +342,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     reserve()
@@ -368,6 +378,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     void
     basic_string<_CharT, _Traits, _Alloc>::
     resize(size_type __n, _CharT __c)
@@ -380,6 +391,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT, _Traits, _Alloc>&
     basic_string<_CharT, _Traits, _Alloc>::
     _M_append(const _CharT* __s, size_type __n)
@@ -400,6 +412,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _CharT, typename _Traits, typename _Alloc>
     template<typename _InputIterator>
+      _GLIBCXX20_CONSTEXPR
       basic_string<_CharT, _Traits, _Alloc>&
       basic_string<_CharT, _Traits, _Alloc>::
       _M_replace_dispatch(const_iterator __i1, const_iterator __i2,
@@ -415,6 +428,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT, _Traits, _Alloc>&
     basic_string<_CharT, _Traits, _Alloc>::
     _M_replace_aux(size_type __pos1, size_type __n1, size_type __n2,
@@ -444,6 +458,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT, _Traits, _Alloc>&
     basic_string<_CharT, _Traits, _Alloc>::
     _M_replace(size_type __pos, size_type __len1, const _CharT* __s,
@@ -459,6 +474,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  pointer __p = this->_M_data() + __pos;
 
 	  const size_type __how_much = __old_size - __pos - __len1;
+#if __cpp_lib_is_constant_evaluated
+	  if (__builtin_is_constant_evaluated())
+	    {
+	      auto __newp = this->_M_get_allocator().allocate(__new_size);
+	      _S_copy(__newp, this->_M_data(), __pos);
+	      _S_copy(__newp + __pos, __s, __len2);
+	      _S_copy(__newp + __pos + __len2, __p + __len1, __how_much);
+	      _S_copy(this->_M_data(), __newp, __new_size);
+	      this->_M_get_allocator().deallocate(__newp, __new_size);
+	    }
+	  else
+#endif
 	  if (_M_disjunct(__s))
 	    {
 	      if (__how_much && __len1 != __len2)
@@ -502,6 +529,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     copy(_CharT* __s, size_type __n, size_type __pos) const
@@ -535,7 +563,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       else
 	__p = _M_data();
       struct _Terminator {
-	~_Terminator() { _M_this->_M_set_length(_M_r); }
+	constexpr ~_Terminator() { _M_this->_M_set_length(_M_r); }
 	basic_string* _M_this;
 	size_type _M_r;
       };
@@ -548,7 +576,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #endif  // _GLIBCXX_USE_CXX11_ABI
    
+#if __cpp_lib_constexpr_string >= 201907L
+# define _GLIBCXX_STRING_CONSTEXPR constexpr
+#else
+# define _GLIBCXX_STRING_CONSTEXPR
+#endif
+
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT, _Traits, _Alloc>
     operator+(const _CharT* __lhs,
 	      const basic_string<_CharT, _Traits, _Alloc>& __rhs)
@@ -569,6 +604,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
     basic_string<_CharT, _Traits, _Alloc>
     operator+(_CharT __lhs, const basic_string<_CharT, _Traits, _Alloc>& __rhs)
     {
@@ -587,6 +623,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find(const _CharT* __s, size_type __pos, size_type __n) const
@@ -623,6 +660,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find(_CharT __c, size_type __pos) const _GLIBCXX_NOEXCEPT
@@ -641,6 +679,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     rfind(const _CharT* __s, size_type __pos, size_type __n) const
@@ -663,6 +702,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     rfind(_CharT __c, size_type __pos) const _GLIBCXX_NOEXCEPT
@@ -680,6 +720,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find_first_of(const _CharT* __s, size_type __pos, size_type __n) const
@@ -696,6 +737,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find_last_of(const _CharT* __s, size_type __pos, size_type __n) const
@@ -718,6 +760,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find_first_not_of(const _CharT* __s, size_type __pos, size_type __n) const
@@ -731,6 +774,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find_first_not_of(_CharT __c, size_type __pos) const _GLIBCXX_NOEXCEPT
@@ -742,6 +786,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find_last_not_of(const _CharT* __s, size_type __pos, size_type __n) const
@@ -764,6 +809,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type
     basic_string<_CharT, _Traits, _Alloc>::
     find_last_not_of(_CharT __c, size_type __pos) const _GLIBCXX_NOEXCEPT
@@ -784,6 +830,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     int
     basic_string<_CharT, _Traits, _Alloc>::
     compare(size_type __pos, size_type __n, const basic_string& __str) const
@@ -799,6 +846,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     int
     basic_string<_CharT, _Traits, _Alloc>::
     compare(size_type __pos1, size_type __n1, const basic_string& __str,
@@ -817,6 +865,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     int
     basic_string<_CharT, _Traits, _Alloc>::
     compare(const _CharT* __s) const _GLIBCXX_NOEXCEPT
@@ -832,6 +881,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     int
     basic_string <_CharT, _Traits, _Alloc>::
     compare(size_type __pos, size_type __n1, const _CharT* __s) const
@@ -848,6 +898,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX_STRING_CONSTEXPR
     int
     basic_string <_CharT, _Traits, _Alloc>::
     compare(size_type __pos, size_type __n1, const _CharT* __s,
@@ -863,6 +914,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __r;
     }
 
+#undef _GLIBCXX_STRING_CONSTEXPR
+
   // 21.3.7.9 basic_string::getline and operators
   template<typename _CharT, typename _Traits, typename _Alloc>
     basic_istream<_CharT, _Traits>&
diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index f0540128364..bafca7bb313 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -35,7 +35,7 @@
 #if ! _GLIBCXX_USE_CXX11_ABI
 
 #ifdef __cpp_lib_is_constant_evaluated
-// Support P1032R1 in C++20 (but not P0980R1 yet).
+// Support P1032R1 in C++20 (but not P0980R1 for COW strings).
 # define __cpp_lib_constexpr_string 201811L
 #elif __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
 // Support P0426R1 changes to char_traits in C++17.
diff --git a/libstdc++-v3/include/ext/type_traits.h b/libstdc++-v3/include/ext/type_traits.h
index fed78d3a527..3b41cf307a4 100644
--- a/libstdc++-v3/include/ext/type_traits.h
+++ b/libstdc++-v3/include/ext/type_traits.h
@@ -148,17 +148,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // For use in string and vstring.
   template<typename _Type>
+    _GLIBCXX_CONSTEXPR
     inline bool
     __is_null_pointer(_Type* __ptr)
     { return __ptr == 0; }
 
   template<typename _Type>
+    _GLIBCXX_CONSTEXPR
     inline bool
     __is_null_pointer(_Type)
     { return false; }
 
 #if __cplusplus >= 201103L
-  inline bool
+  constexpr bool
   __is_null_pointer(std::nullptr_t)
   { return true; }
 #endif
diff --git a/libstdc++-v3/include/std/string b/libstdc++-v3/include/std/string
index af840e887d5..1c5d1800c15 100644
--- a/libstdc++-v3/include/std/string
+++ b/libstdc++-v3/include/std/string
@@ -116,6 +116,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _CharT, typename _Traits, typename _Alloc,
 	   typename _Predicate>
+    _GLIBCXX20_CONSTEXPR
     inline typename basic_string<_CharT, _Traits, _Alloc>::size_type
     erase_if(basic_string<_CharT, _Traits, _Alloc>& __cont, _Predicate __pred)
     {
@@ -129,6 +130,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc, typename _Up>
+    _GLIBCXX20_CONSTEXPR
     inline typename basic_string<_CharT, _Traits, _Alloc>::size_type
     erase(basic_string<_CharT, _Traits, _Alloc>& __cont, const _Up& __value)
     {
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index 0930de82efa..fd7e1258543 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -232,7 +232,11 @@
 #define __cpp_lib_constexpr_memory 201811L
 #define __cpp_lib_constexpr_numeric 201911L
 #ifdef __cpp_lib_is_constant_evaluated
-# define __cpp_lib_constexpr_string 201811L
+# if _GLIBCXX_USE_CXX11_ABI
+#  define __cpp_lib_constexpr_string 201907L
+# else
+#  define __cpp_lib_constexpr_string 201811L
+# endif
 #endif
 #define __cpp_lib_constexpr_string_view 201811L
 #define __cpp_lib_constexpr_tuple 201811L
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc
new file mode 100644
index 00000000000..191ff1cb379
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/constexpr.cc
@@ -0,0 +1,174 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <string>
+
+#ifndef __cpp_lib_constexpr_string
+# error "Feature-test macro for constexpr std::string missing in <string>"
+#elif __cpp_lib_constexpr_string != 201907L
+# error "Feature-test macro for constexpr std::string has wrong value in <string>"
+#endif
+
+#include <testsuite_hooks.h>
+
+using C = char;
+using T = std::char_traits<C>;
+
+template<typename T>
+struct Alloc : std::allocator<T>
+{
+  using std::allocator<T>::allocator;
+
+  constexpr explicit Alloc(int p) : personality(p) { }
+
+  template<typename U>
+    constexpr Alloc(const Alloc<U>& a) : personality(a.personality) { }
+
+  int personality = 0;
+
+  constexpr Alloc select_on_container_copy_construction() const
+  { return Alloc(-1); }
+
+  constexpr bool operator==(const Alloc& a) const noexcept
+  { return personality == a.personality; }
+};
+
+constexpr bool
+test_default_ctor()
+{
+  std::basic_string<C> s0;
+  VERIFY( s0.empty() );
+
+  std::basic_string<C> s1(std::allocator<C>{});
+  VERIFY( s1.empty() );
+
+  std::basic_string<C, T, Alloc<C>> s2;
+  VERIFY( s2.empty() );
+
+  std::basic_string<C, T, Alloc<C>> s3(Alloc<C>(3));
+  VERIFY( s3.empty() );
+  VERIFY( s3.get_allocator().personality == 3 );
+
+  return true;
+}
+
+static_assert( test_default_ctor() );
+
+constexpr bool
+test_cstr()
+{
+  const C cs[] = "This has an embedded \0 null";
+  const auto len = (sizeof(cs) - 1)/sizeof(C);
+
+  std::basic_string<C> s1(cs);
+  VERIFY( s1.length() == 21 );
+  std::basic_string<C> s2(cs, std::allocator<C>{});
+  VERIFY( s2 == s1 );
+
+  std::basic_string<C> s3(cs, len);
+  VERIFY( s3.length() == len );
+  VERIFY( s3[len] == '\0' );
+  std::basic_string<C> s4(cs, len, std::allocator<C>{});
+  VERIFY( s4 == s3 );
+
+  std::basic_string<C, T, Alloc<C>> s5(cs);
+  VERIFY( s5 == std::basic_string_view<C>(s1) );
+
+  std::basic_string<C, T, Alloc<C>> s6(cs, Alloc<C>(6));
+  VERIFY( s6 == std::basic_string_view<C>(s1) );
+  VERIFY( s6.get_allocator().personality == 6 );
+
+  std::basic_string<C, T, Alloc<C>> s7(cs, len, Alloc<C>(7));
+  VERIFY( s7 == std::basic_string_view<C>(s3) );
+  VERIFY( s7.get_allocator().personality == 7 );
+
+  return true;
+}
+
+static_assert( test_cstr() );
+
+constexpr bool
+test_copy()
+{
+  const std::basic_string<C> short_string = "sh";
+  const std::basic_string<C> long_string = "string longer than the SSO buffer";
+
+  std::basic_string<C> s1 = short_string;
+  VERIFY( s1 == short_string );
+  std::basic_string<C> s2(short_string, s1.get_allocator());
+  VERIFY( s2 == short_string );
+
+  std::basic_string<C> s3 = long_string;
+  VERIFY( s3 == long_string );
+  std::basic_string<C> s4(long_string, s1.get_allocator());
+  VERIFY( s4 == long_string );
+
+  std::basic_string<C, T, Alloc<C>> a_short_string = short_string.c_str();
+  std::basic_string<C, T, Alloc<C>> a_long_string = long_string.c_str();
+
+  std::basic_string<C, T, Alloc<C>> s5(a_short_string);
+  VERIFY( s5 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s6(a_short_string, s5.get_allocator());
+  VERIFY( s6 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s7(a_short_string, Alloc<C>(7));
+  VERIFY( s7 == a_short_string );
+  VERIFY( s7.get_allocator().personality == 7 );
+
+  std::basic_string<C, T, Alloc<C>> s8 = a_long_string;
+  VERIFY( s8 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s9(a_long_string, s5.get_allocator());
+  VERIFY( s9 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s10(a_long_string, Alloc<C>(10));
+  VERIFY( s10 == a_long_string );
+  VERIFY( s10.get_allocator().personality == 10 );
+
+  return true;
+}
+
+static_assert( test_copy() );
+
+constexpr bool
+test_move()
+{
+  const std::basic_string<C> short_string = "sh";
+  const std::basic_string<C> long_string = "string longer than the SSO buffer";
+
+  std::basic_string<C> s0 = short_string;
+
+  std::basic_string<C> s1 = std::move(s0);
+  VERIFY( s1 == short_string );
+  std::basic_string<C> s2(std::move(s1), std::allocator<C>());
+  VERIFY( s2 == short_string );
+
+  s0 = long_string;
+  std::basic_string<C> s3 = std::move(s0);
+  VERIFY( s3 == long_string );
+  std::basic_string<C> s4(std::move(s3), s1.get_allocator());
+  VERIFY( s4 == long_string );
+
+  std::basic_string<C, T, Alloc<C>> a_short_string = short_string.c_str();
+  std::basic_string<C, T, Alloc<C>> a_long_string = long_string.c_str();
+
+  auto sa0 = a_short_string;
+  std::basic_string<C, T, Alloc<C>> s5 = std::move(sa0);
+  VERIFY( s5 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s6(std::move(s5), sa0.get_allocator());
+  VERIFY( s6 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s7(std::move(s6), Alloc<C>(7));
+  VERIFY( s7 == a_short_string );
+  VERIFY( s7.get_allocator().personality == 7 );
+
+  sa0 = a_long_string;
+  std::basic_string<C, T, Alloc<C>> s8 = std::move(sa0);
+  VERIFY( s8 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s9(std::move(s8), s5.get_allocator());
+  VERIFY( s9 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s10(std::move(s9), Alloc<C>(10));
+  VERIFY( s10 == a_long_string );
+  VERIFY( s10.get_allocator().personality == 10 );
+
+  return true;
+}
+
+static_assert( test_move() );
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc
new file mode 100644
index 00000000000..3ef9862e613
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc
@@ -0,0 +1,174 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <string>
+
+#ifndef __cpp_lib_constexpr_string
+# error "Feature-test macro for constexpr std::string missing in <string>"
+#elif __cpp_lib_constexpr_string != 201907L
+# error "Feature-test macro for constexpr std::string has wrong value in <string>"
+#endif
+
+#include <testsuite_hooks.h>
+
+using C = wchar_t;
+using T = std::char_traits<C>;
+
+template<typename T>
+struct Alloc : std::allocator<T>
+{
+  using std::allocator<T>::allocator;
+
+  constexpr explicit Alloc(int p) : personality(p) { }
+
+  template<typename U>
+    constexpr Alloc(const Alloc<U>& a) : personality(a.personality) { }
+
+  int personality = 0;
+
+  constexpr Alloc select_on_container_copy_construction() const
+  { return Alloc(-1); }
+
+  constexpr bool operator==(const Alloc& a) const noexcept
+  { return personality == a.personality; }
+};
+
+constexpr bool
+test_default_ctor()
+{
+  std::basic_string<C> s0;
+  VERIFY( s0.empty() );
+
+  std::basic_string<C> s1(std::allocator<C>{});
+  VERIFY( s1.empty() );
+
+  std::basic_string<C, T, Alloc<C>> s2;
+  VERIFY( s2.empty() );
+
+  std::basic_string<C, T, Alloc<C>> s3(Alloc<C>(3));
+  VERIFY( s3.empty() );
+  VERIFY( s3.get_allocator().personality == 3 );
+
+  return true;
+}
+
+static_assert( test_default_ctor() );
+
+constexpr bool
+test_cstr()
+{
+  const C cs[] = L"This has an embedded \0 null";
+  const auto len = (sizeof(cs) - 1)/sizeof(C);
+
+  std::basic_string<C> s1(cs);
+  VERIFY( s1.length() == 21 );
+  std::basic_string<C> s2(cs, std::allocator<C>{});
+  VERIFY( s2 == s1 );
+
+  std::basic_string<C> s3(cs, len);
+  VERIFY( s3.length() == len );
+  VERIFY( s3[len] == L'\0' );
+  std::basic_string<C> s4(cs, len, std::allocator<C>{});
+  VERIFY( s4 == s3 );
+
+  std::basic_string<C, T, Alloc<C>> s5(cs);
+  VERIFY( s5 == std::basic_string_view<C>(s1) );
+
+  std::basic_string<C, T, Alloc<C>> s6(cs, Alloc<C>(6));
+  VERIFY( s6 == std::basic_string_view<C>(s1) );
+  VERIFY( s6.get_allocator().personality == 6 );
+
+  std::basic_string<C, T, Alloc<C>> s7(cs, len, Alloc<C>(7));
+  VERIFY( s7 == std::basic_string_view<C>(s3) );
+  VERIFY( s7.get_allocator().personality == 7 );
+
+  return true;
+}
+
+static_assert( test_cstr() );
+
+constexpr bool
+test_copy()
+{
+  const std::basic_string<C> short_string = L"sh";
+  const std::basic_string<C> long_string = L"string longer than the SSO buffer";
+
+  std::basic_string<C> s1 = short_string;
+  VERIFY( s1 == short_string );
+  std::basic_string<C> s2(short_string, s1.get_allocator());
+  VERIFY( s2 == short_string );
+
+  std::basic_string<C> s3 = long_string;
+  VERIFY( s3 == long_string );
+  std::basic_string<C> s4(long_string, s1.get_allocator());
+  VERIFY( s4 == long_string );
+
+  std::basic_string<C, T, Alloc<C>> a_short_string = short_string.c_str();
+  std::basic_string<C, T, Alloc<C>> a_long_string = long_string.c_str();
+
+  std::basic_string<C, T, Alloc<C>> s5(a_short_string);
+  VERIFY( s5 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s6(a_short_string, s5.get_allocator());
+  VERIFY( s6 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s7(a_short_string, Alloc<C>(7));
+  VERIFY( s7 == a_short_string );
+  VERIFY( s7.get_allocator().personality == 7 );
+
+  std::basic_string<C, T, Alloc<C>> s8 = a_long_string;
+  VERIFY( s8 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s9(a_long_string, s5.get_allocator());
+  VERIFY( s9 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s10(a_long_string, Alloc<C>(10));
+  VERIFY( s10 == a_long_string );
+  VERIFY( s10.get_allocator().personality == 10 );
+
+  return true;
+}
+
+static_assert( test_copy() );
+
+constexpr bool
+test_move()
+{
+  const std::basic_string<C> short_string = L"sh";
+  const std::basic_string<C> long_string = L"string longer than the SSO buffer";
+
+  std::basic_string<C> s0 = short_string;
+
+  std::basic_string<C> s1 = std::move(s0);
+  VERIFY( s1 == short_string );
+  std::basic_string<C> s2(std::move(s1), std::allocator<C>());
+  VERIFY( s2 == short_string );
+
+  s0 = long_string;
+  std::basic_string<C> s3 = std::move(s0);
+  VERIFY( s3 == long_string );
+  std::basic_string<C> s4(std::move(s3), s1.get_allocator());
+  VERIFY( s4 == long_string );
+
+  std::basic_string<C, T, Alloc<C>> a_short_string = short_string.c_str();
+  std::basic_string<C, T, Alloc<C>> a_long_string = long_string.c_str();
+
+  auto sa0 = a_short_string;
+  std::basic_string<C, T, Alloc<C>> s5 = std::move(sa0);
+  VERIFY( s5 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s6(std::move(s5), sa0.get_allocator());
+  VERIFY( s6 == a_short_string );
+  std::basic_string<C, T, Alloc<C>> s7(std::move(s6), Alloc<C>(7));
+  VERIFY( s7 == a_short_string );
+  VERIFY( s7.get_allocator().personality == 7 );
+
+  sa0 = a_long_string;
+  std::basic_string<C, T, Alloc<C>> s8 = std::move(sa0);
+  VERIFY( s8 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s9(std::move(s8), s5.get_allocator());
+  VERIFY( s9 == a_long_string );
+  std::basic_string<C, T, Alloc<C>> s10(std::move(s9), Alloc<C>(10));
+  VERIFY( s10 == a_long_string );
+  VERIFY( s10.get_allocator().personality == 10 );
+
+  return true;
+}
+
+static_assert( test_move() );
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/literals/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/literals/constexpr.cc
new file mode 100644
index 00000000000..27d286c85eb
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/literals/constexpr.cc
@@ -0,0 +1,22 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <string>
+#include <testsuite_hooks.h>
+
+constexpr bool
+test_literals()
+{
+  using namespace std::literals;
+
+  auto s = "narrow string"s;
+  auto sw = L"wide string"s;
+  auto s8 = u8"UTF-8 string"s;
+  auto su = u"UTF-16 string"s;
+  auto sU = U"UTF-32 string"s;
+
+  return !s.empty() && !sw.empty() && !s8.empty() && !su.empty() && !sU.empty();
+}
+
+static_assert( test_literals() );
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
new file mode 100644
index 00000000000..c875a3a19ad
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
@@ -0,0 +1,52 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <string>
+#include <testsuite_hooks.h>
+
+constexpr bool
+test_insert()
+{
+  std::string s;
+  s.insert(0, "one");
+  VERIFY( s == "one" );
+  s.insert(0, "eleventy-");
+  VERIFY( s == "eleventy-one" );
+  s.insert(6, "ses at ten thirteen", 15);
+  VERIFY( s == "elevenses at ten thirty-one" );
+
+  return true;
+}
+
+static_assert( test_insert() );
+
+constexpr bool
+test_replace()
+{
+  std::string s = "abcdef";
+  s.replace(2, 1, s.c_str(), 3);
+  VERIFY( s == "ababcdef" );
+  s.replace(0, 2, "", 0);
+  VERIFY( s == "abcdef" );
+  s.replace(1, 4, "ardwol", 6);
+  VERIFY( s == "aardwolf" );
+  s.replace(4, 0, "vark not wolf");
+
+  return true;
+}
+
+static_assert( test_replace() );
+
+constexpr bool
+test_erasure()
+{
+  std::string s = "Spiritualized Electric Mainline";
+  std::erase(s, 'i');
+  VERIFY( s == "Sprtualzed Electrc Manlne" );
+  std::erase_if(s, [](char c) { return c == 'l'; });
+  VERIFY( s == "Sprtuazed Eectrc Manne" );
+
+  return true;
+}
+
+static_assert( test_erasure() );
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc
new file mode 100644
index 00000000000..9fd05c9b880
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc
@@ -0,0 +1,49 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <string>
+#include <testsuite_hooks.h>
+
+using C = char;
+using T = std::char_traits<C>;
+
+constexpr bool
+test_swap()
+{
+  std::basic_string<C> s0;
+  s0.swap(s0);
+  VERIFY( s0.empty() );
+  std::basic_string<C> s00;
+  s0.swap(s00);
+  VERIFY( s0.empty() );
+  VERIFY( s00.empty() );
+
+  std::basic_string<C> s1 = "s1";
+  s1.swap(s0);
+  VERIFY( s1.empty() );
+  VERIFY( ! s0.empty() );
+  s1.swap(s0);
+  VERIFY( s0.empty() );
+  VERIFY( ! s1.empty() );
+
+  std::basic_string<C> s2 = "quite a long string, but not very long";
+  s2.swap(s1);
+  VERIFY( s2.length() == 2 );
+  VERIFY( s1.length() == 38 );
+  s2.swap(s1);
+  VERIFY( s1.length() == 2 );
+  VERIFY( s2.length() == 38 );
+
+  swap(s2, s0);
+  VERIFY( s2.empty() );
+  VERIFY( s0.length() == 38 );
+
+  auto s3 = s0;
+  swap(s3, s0);
+  VERIFY( s3 == s0 );
+
+  return true;
+}
+
+static_assert( test_swap() );
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc
new file mode 100644
index 00000000000..4d3f188f1d6
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc
@@ -0,0 +1,49 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <string>
+#include <testsuite_hooks.h>
+
+using C = wchar_t;
+using T = std::char_traits<C>;
+
+constexpr bool
+test_swap()
+{
+  std::basic_string<C> s0;
+  s0.swap(s0);
+  VERIFY( s0.empty() );
+  std::basic_string<C> s00;
+  s0.swap(s00);
+  VERIFY( s0.empty() );
+  VERIFY( s00.empty() );
+
+  std::basic_string<C> s1 = L"s1";
+  s1.swap(s0);
+  VERIFY( s1.empty() );
+  VERIFY( ! s0.empty() );
+  s1.swap(s0);
+  VERIFY( s0.empty() );
+  VERIFY( ! s1.empty() );
+
+  std::basic_string<C> s2 = L"quite a long string, but not very long";
+  s2.swap(s1);
+  VERIFY( s2.length() == 2 );
+  VERIFY( s1.length() == 38 );
+  s2.swap(s1);
+  VERIFY( s1.length() == 2 );
+  VERIFY( s2.length() == 38 );
+
+  swap(s2, s0);
+  VERIFY( s2.empty() );
+  VERIFY( s0.length() == 38 );
+
+  auto s3 = s0;
+  swap(s3, s0);
+  VERIFY( s3 == s0 );
+
+  return true;
+}
+
+static_assert( test_swap() );
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/version.cc b/libstdc++-v3/testsuite/21_strings/basic_string/version.cc
new file mode 100644
index 00000000000..a99bb179a93
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/version.cc
@@ -0,0 +1,25 @@
+// { dg-do compile { target c++17 } }
+
+#include <version>
+
+#ifndef __cpp_lib_constexpr_string
+# error "Feature-test macro for constexpr std::string missing in <version>"
+#endif
+
+#if __cplusplus == 201703L
+# if __cpp_lib_constexpr_string != 201611L
+#  error "Feature-test macro for constexpr std::string has wrong value for C++17 in <version>"
+# endif
+#endif
+
+#if __cplusplus == 202002L
+# if _GLIBCXX_USE_CXX11_ABI
+#  if __cpp_lib_constexpr_string != 201907L
+#   error "Feature-test macro for constexpr std::string has wrong value for C++20 in <version>"
+#  endif
+# else // COW strings
+#  if __cpp_lib_constexpr_string != 201811L
+#   error "Feature-test macro for constexpr std::string has wrong value for C++20 in <version>"
+#  endif
+# endif
+#endif
-- 
2.31.1



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