Clang "error: requires clause differs in template redeclaration" on out-of-class definition of class template specialization member

[stbergmann]

At least with recent Clang 17 trunk:

$ cat test.cc
template<typename T> concept C1 = requires(T t) { --t; };
template<typename T> concept C2 = C1<T> && requires(T t) { *t; };
template<typename T> requires C1<T> struct S { void f(); };
template<typename T> requires C2<T> struct S<T> { void f(); };
template<typename T> requires C1<T> void S<T>::f() {}
template<typename T> requires C2<T> void S<T>::f() {}

$ clang++ -std=c++20 -fsyntax-only test.cc
test.cc:22:48: error: redefinition of 'f'
template<typename T> requires C1<T> void S<T>::f() {}
                                               ^
test.cc:21:48: note: previous definition is here
template<typename T> requires C1<T> void S<T>::f() {}
                                               ^
1 error generated.

(This causes issues with libstdc++, see https://gcc.gnu.org/pipermail/libstdc++/2023-February/055442.html "Re: <ranges> error: requires clause differs in template redeclaration".)

[jwakely]

Dup of #58124 which is a dup of #49620

[llvmbot]

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[llvmbot]

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