[PATCH] PR c++/91369 Implement P0784R7 changes to allocation and construction

[Stephan Bergmann]

On 23/10/2019 21:27, Jonathan Wakely wrote:
> This patch is the first part of library support for constexpr
> std::vector and std::string. This only includes the changes to
> std::allocator, std::allocator_traits, std::construct_at,
> std::destroy_at, std::destroy and std::destroy_n.
[...]
> Tested x86_64-linux and powerpc64le-linux, for every -std=gnu++NN
> mode.

Clang (trunk, at least) with -std=c++11 now complains about some of the 
constexpr (because they return void, or don't have a return statement) 
when including <memory>.

What fixed it for me locally:

> diff --git a/libstdc++-v3/include/bits/alloc_traits.h b/libstdc++-v3/include/bits/alloc_traits.h
> index 26d6d26ae48..61d3c1b794b 100644
> --- a/libstdc++-v3/include/bits/alloc_traits.h
> +++ b/libstdc++-v3/include/bits/alloc_traits.h
> @@ -241,13 +241,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  = typename __construct_helper<_Tp, _Args...>::type;
>  
>        template<typename _Tp, typename... _Args>
> -	static constexpr _Require<__has_construct<_Tp, _Args...>>
> +	static
> +#if __cplusplus >= 201402L
> +	       constexpr
> +#endif
> +	                 _Require<__has_construct<_Tp, _Args...>>
>  	_S_construct(_Alloc& __a, _Tp* __p, _Args&&... __args)
>  	noexcept(noexcept(__a.construct(__p, std::forward<_Args>(__args)...)))
>  	{ __a.construct(__p, std::forward<_Args>(__args)...); }
>  
>        template<typename _Tp, typename... _Args>
> -	static constexpr
> +	static
> +#if __cplusplus >= 201402L
> +	       constexpr
> +#endif
>  	_Require<__and_<__not_<__has_construct<_Tp, _Args...>>,
>  			       is_constructible<_Tp, _Args...>>>
>  	_S_construct(_Alloc&, _Tp* __p, _Args&&... __args)
> @@ -256,14 +263,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	{ std::_Construct(__p, std::forward<_Args>(__args)...); }
>  
>        template<typename _Alloc2, typename _Tp>
> -	static constexpr auto
> +	static
> +#if __cplusplus >= 201402L
> +	       constexpr
> +#endif
> +	                 auto
>  	_S_destroy(_Alloc2& __a, _Tp* __p, int)
>  	noexcept(noexcept(__a.destroy(__p)))
>  	-> decltype(__a.destroy(__p))
>  	{ __a.destroy(__p); }
>  
>        template<typename _Alloc2, typename _Tp>
> -	static constexpr void
> +	static
> +#if __cplusplus >= 201402L
> +	       constexpr
> +#endif
> +	                 void
>  	_S_destroy(_Alloc2&, _Tp* __p, ...)
>  	noexcept(noexcept(__p->~_Tp()))
>  	{ std::_Destroy(__p); }
> @@ -562,7 +577,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #endif
>  
>    template<typename _Alloc>
> -    constexpr void
> +#if __cplusplus >= 201402L
> +    constexpr
> +#endif
> +              void
>      __alloc_on_copy(_Alloc& __one, const _Alloc& __two)
>      {
>        typedef allocator_traits<_Alloc> __traits;
> @@ -594,7 +612,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #endif
>  
>    template<typename _Alloc>
> -    constexpr void
> +#if __cplusplus >= 201402L
> +    constexpr
> +#endif
> +              void
>      __alloc_on_move(_Alloc& __one, _Alloc& __two)
>      {
>        typedef allocator_traits<_Alloc> __traits;
> @@ -621,7 +642,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #endif
>  
>    template<typename _Alloc>
> -    constexpr void
> +#if __cplusplus >= 201402L
> +    constexpr
> +#endif
> +              void
>      __alloc_on_swap(_Alloc& __one, _Alloc& __two)
>      {
>        typedef allocator_traits<_Alloc> __traits;
> diff --git a/libstdc++-v3/include/bits/stl_construct.h b/libstdc++-v3/include/bits/stl_construct.h
> index a16196ffe74..ed9b72b7327 100644
> --- a/libstdc++-v3/include/bits/stl_construct.h
> +++ b/libstdc++-v3/include/bits/stl_construct.h
> @@ -137,7 +137,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     * Destroy the object pointed to by a pointer type.
>     */
>    template<typename _Tp>
> -    _GLIBCXX_CONSTEXPR inline void
> +#if __cplusplus >= 201402L
> +    _GLIBCXX_CONSTEXPR
> +#endif
> +                       inline void
>      _Destroy(_Tp* __pointer)
>      {
>  #if __cplusplus > 201703L
> diff --git a/libstdc++-v3/include/ext/alloc_traits.h b/libstdc++-v3/include/ext/alloc_traits.h
> index 052d811ec01..ddeda66b57a 100644
> --- a/libstdc++-v3/include/ext/alloc_traits.h
> +++ b/libstdc++-v3/include/ext/alloc_traits.h
> @@ -76,7 +76,11 @@ template<typename _Alloc, typename = typename _Alloc::value_type>
>    public:
>      // overload construct for non-standard pointer types
>      template<typename _Ptr, typename... _Args>
> -      static constexpr std::__enable_if_t<__is_custom_pointer<_Ptr>::value>
> +      static
> +#if __cplusplus >= 201402L
> +             constexpr
> +#endif
> +                       std::__enable_if_t<__is_custom_pointer<_Ptr>::value>
>        construct(_Alloc& __a, _Ptr __p, _Args&&... __args)
>        noexcept(noexcept(_Base_type::construct(__a, std::__to_address(__p),
>  					      std::forward<_Args>(__args)...)))
> @@ -87,7 +91,11 @@ template<typename _Alloc, typename = typename _Alloc::value_type>
>  
>      // overload destroy for non-standard pointer types
>      template<typename _Ptr>
> -      static constexpr std::__enable_if_t<__is_custom_pointer<_Ptr>::value>
> +      static
> +#if __cplusplus >= 201402L
> +             constexpr
> +#endif
> +                       std::__enable_if_t<__is_custom_pointer<_Ptr>::value>
>        destroy(_Alloc& __a, _Ptr __p)
>        noexcept(noexcept(_Base_type::destroy(__a, std::__to_address(__p))))
>        { _Base_type::destroy(__a, std::__to_address(__p)); }
> @@ -95,7 +103,11 @@ template<typename _Alloc, typename = typename _Alloc::value_type>
>      static constexpr _Alloc _S_select_on_copy(const _Alloc& __a)
>      { return _Base_type::select_on_container_copy_construction(__a); }
>  
> -    static constexpr void _S_on_swap(_Alloc& __a, _Alloc& __b)
> +    static
> +#if __cplusplus >= 201402L
> +           constexpr
> +#endif
> +                     void _S_on_swap(_Alloc& __a, _Alloc& __b)
>      { std::__alloc_on_swap(__a, __b); }
>  
>      static constexpr bool _S_propagate_on_copy_assign()