[PATCH] libstdc++: Constrain std::swap using concepts in C++20

[Jonathan Wakely]

On Tue, 15 Jul 2025 at 08:29, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Mon, Jul 14, 2025 at 10:38 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> This is a minor compile-time optimization for C++20.
>
> Please mention that you also replaced  _GLIBCXX20_CONSTEXPR, with
> constexpr under __glibcxx_concepts (that is >= c++ 20).

I've just discovered that the last non-Clang-based version of Intel
icc did not define __cpp_concepts in its C++20 mode:
https://godbolt.org/z/vbTzjxoro
And so __glibcxx_concepts won't be defined for that compiler.

Sigh. This means std::swap will no longer be constexpr in C++20 mode,
so I might put the _GLIBCXX20_CONSTEXPR back in the #elif __cplusplus
>= 201103L group.