clang rejects valid friend template function declaration with noexcept specifier in template class

[Rush10233]

The following code is rejected by clang for the mismatch of the noexcept specifier between the friend declaration and definition of func:

template <typename T>
struct C {
    template <int N>
    friend void func() noexcept(N == 0);
};

template <int N>
void func() noexcept(N == 0) {}

int main() {
    C<int> t;
    return 0;
}

The diagnostic says that:

<source>:4:17: error: exception specification in declaration does not match the previous declaration
    4 |     friend void func() noexcept(N == 0);
      |                 ^
<source>:11:12: note: in instantiation of template class 'C<int>' requested here
   11 |     C<int> t;
      |            ^
<source>:8:6: note: previous declaration is here
    8 | void func() noexcept(N == 0) {}

That is quite unreasonable. It should be clear that the noexcept expression of func keeps identical between the friend declaration and definition statement. Removing the dependent name N in the noexcept expression or replacing template class C with a non-template one can eliminate the diagnostic.

https://godbolt.org/z/rPh84Phzn

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Rush10233)

The following code is rejected by clang for the mismatch of the `noexcept` specifier between the friend declaration and definition of `func`:

template <typename T>
struct C {
    template <int N>
    friend void func() noexcept(N == 0);
};

template <int N>
void func() noexcept(N == 0) {}

int main() {
    C<int> t;
    return 0;
}

The diagnostic says that:

<source>:4:17: error: exception specification in declaration does not match the previous declaration
    4 |     friend void func() noexcept(N == 0);
      |                 ^
<source>:11:12: note: in instantiation of template class 'C<int>' requested here
   11 |     C<int> t;
      |            ^
<source>:8:6: note: previous declaration is here
    8 | void func() noexcept(N == 0) {}

That is quite unreasonable. It should be clear that the noexcept expression of func keeps identical between the friend declaration and definition statement. Removing the dependent name N in the noexcept expression or replacing template class C with a non-template one can eliminate the diagnostic.

https://godbolt.org/z/rPh84Phzn