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From: "ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/92944] [concepts] redefinition error when using constrained
 structure template inside namespace
Date: Tue, 14 Dec 2021 11:22:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D92944

--- Comment #1 from Ed Catmur <ed at catmur dot uk> ---
This becomes problematic when N::Q is std::hash; we are encouraged not to
reopen namespace std but to specialize std::hash from the root namespace.

Example:

#include <functional>
template<class> constexpr bool P =3D false;
template<class> constexpr bool Q =3D false;
template<class T> requires P<T> struct std::hash<T> { std::size_t operator(=
)(T)
{ return 1; } };
template<class U> requires Q<U> struct std::hash<U> { std::size_t operator(=
)(U)
{ return 2; } };
struct S {};
template<> constexpr bool P<S> =3D true;
int i =3D std::hash<S>()(S());

clang and MSVC correctly accept this.=