Why isn't this class specialization using a concept accepted?

Question

The following code attempts to partially specialize a class using a concept and add a method to the specialization, but it is rejected by clang 11.0.0:

#include <concepts>

template <typename T> // note: previous template declaration is here
struct S {};

template <std::integral T>
struct S<T>
{
    void f();
};

template <std::integral T> // error: type constraint differs in template redeclaration
void S<T>::f()
{
}

clang gives the error message:

<source>:14:16: error: type constraint differs in template redeclaration
template <std::integral T>
               ^
<source>:3:11: note: previous template declaration is here
template <typename T>

(see https://godbolt.org/z/Wv1ojK). Why is this code wrong? Or is this a bug in clang? (FWIW, this code is accepted by gcc trunk and by MSVC 19.28, although that's no guarantee of correctness.)

Answer 1

This is definitely a CLANG bug. It is already filed - #48020.