[{"id":3683531,"web_url":"http://patchwork.ozlabs.org/comment/3683531/","msgid":"","list_archive_url":null,"date":"2026-04-28T15:04:04","subject":"Re: [PATCH] libstdc++: Make pointer_traits::pointer_to constexpr for\n main template.","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/people/48004/","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"content":"On Tue, 28 Apr 2026 at 15:41, Tomasz Kamiński wrote:\n>\n> This resolves LWG3454, \"pointer_traits::pointer_to should be constexpr\",\n> accepted in Kona 2025.\n>\n> The change is applied since C++20, i.e. standard in which pointer_to\n> was made constexpr for T* specialization.\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/bits/ptr_traits.h (__ptr_traits_ptr_to::pointer_to):\n> Define as constexpr since C++20.\n> * testsuite/20_util/pointer_traits/pointer_to_constexpr.cc:\n> New test for custom pointer-like type.\n> ---\n> Tested on x86_64-linux locally. OK for trunk?\n\nOK\n\n\n>\n> libstdc++-v3/include/bits/ptr_traits.h | 4 +++-\n> .../20_util/pointer_traits/pointer_to_constexpr.cc | 14 ++++++++++++++\n> 2 files changed, 17 insertions(+), 1 deletion(-)\n>\n> diff --git a/libstdc++-v3/include/bits/ptr_traits.h b/libstdc++-v3/include/bits/ptr_traits.h\n> index df83e0cad1d..c1dc76509ce 100644\n> --- a/libstdc++-v3/include/bits/ptr_traits.h\n> +++ b/libstdc++-v3/include/bits/ptr_traits.h\n> @@ -97,13 +97,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> using pointer = _Ptr;\n> using element_type = _Elt;\n>\n> + // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> + // 3454. pointer_traits::pointer_to should be constexpr\n> /**\n> * @brief Obtain a pointer to an object\n> * @param __r A reference to an object of type `element_type`\n> * @return `pointer::pointer_to(__r)`\n> * @pre `pointer::pointer_to(__r)` is a valid expression.\n> */\n> - static pointer\n> + static _GLIBCXX20_CONSTEXPR pointer\n> pointer_to(element_type& __r)\n> #if __cpp_lib_concepts\n> requires requires {\n> 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\n> index cfd35f899c4..929d004fd4a 100644\n> --- a/libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.cc\n> +++ b/libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.cc\n> @@ -24,3 +24,17 @@ static_assert( std::pointer_traits::pointer_to(i) == &i );\n>\n> struct X { } x;\n> static_assert( std::pointer_traits::pointer_to(x) == &x );\n> +\n> +template\n> +struct Ptr\n> +{\n> + T* value;\n> +\n> + constexpr static Ptr\n> + pointer_to(T& t) { return Ptr{&t}; }\n> +\n> + friend bool operator==(Ptr, Ptr) = default;\n> +};\n> +\n> +static_assert( std::pointer_traits>::pointer_to(i) == Ptr{&i} );\n> +static_assert( std::pointer_traits>::pointer_to(x) == Ptr{&x} );\n> --\n> 2.53.0\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=J+Sw1F7b;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=J+Sw1F7b","sourceware.org; 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