[PATCH] libstdc++: Make pointer_traits::pointer_to constexpr for main template.

[Jonathan Wakely]

On Tue, 28 Apr 2026 at 15:41, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
> This resolves LWG3454, "pointer_traits::pointer_to should be constexpr",
> accepted in Kona 2025.
>
> The change is applied since C++20, i.e. standard in which pointer_to
> was made constexpr for T* specialization.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/ptr_traits.h (__ptr_traits_ptr_to::pointer_to):
>         Define as constexpr since C++20.
>         * testsuite/20_util/pointer_traits/pointer_to_constexpr.cc:
>         New test for custom pointer-like type.
> ---
> Tested on x86_64-linux locally. OK for trunk?

OK


>
>  libstdc++-v3/include/bits/ptr_traits.h             |  4 +++-
>  .../20_util/pointer_traits/pointer_to_constexpr.cc | 14 ++++++++++++++
>  2 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/ptr_traits.h b/libstdc++-v3/include/bits/ptr_traits.h
> index df83e0cad1d..c1dc76509ce 100644
> --- a/libstdc++-v3/include/bits/ptr_traits.h
> +++ b/libstdc++-v3/include/bits/ptr_traits.h
> @@ -97,13 +97,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        using pointer = _Ptr;
>        using element_type = _Elt;
>
> +      // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +      // 3454. pointer_traits::pointer_to should be constexpr
>        /**
>         *  @brief  Obtain a pointer to an object
>         *  @param  __r  A reference to an object of type `element_type`
>         *  @return `pointer::pointer_to(__r)`
>         *  @pre `pointer::pointer_to(__r)` is a valid expression.
>        */
> -      static pointer
> +      static _GLIBCXX20_CONSTEXPR pointer
>        pointer_to(element_type& __r)
>  #if __cpp_lib_concepts
>        requires requires {
> diff --git a/libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.cc b/libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.cc
> index cfd35f899c4..929d004fd4a 100644
> --- a/libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.cc
> +++ b/libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.cc
> @@ -24,3 +24,17 @@ static_assert( std::pointer_traits<int*>::pointer_to(i) == &i );
>
>  struct X { } x;
>  static_assert( std::pointer_traits<X*>::pointer_to(x) == &x );
> +
> +template<typename T>
> +struct Ptr
> +{
> +  T* value;
> +
> +  constexpr static Ptr
> +  pointer_to(T& t) { return Ptr{&t}; }
> +
> +  friend bool operator==(Ptr, Ptr) = default;
> +};
> +
> +static_assert( std::pointer_traits<Ptr<int>>::pointer_to(i) == Ptr<int>{&i} );
> +static_assert( std::pointer_traits<Ptr<X>>::pointer_to(x) == Ptr<X>{&x} );
> --
> 2.53.0
>