Cannot declare friend function template with unqualified name in C++20 mode without prior forward declaration

[Fedr]

This program

template <typename T>
class A {
    friend void f<>(A);
};

template <typename T>
void f(A<T>) {}

int main() {
    f(A<int>{});
}

is accepted in GCC and MSVC starting from C++20. But Clang keeps complaining:

error: no candidate function template was found for dependent friend function template specialization
    3 |     friend void f<>(A);
      |                 ^

Online demo: https://gcc.godbolt.org/z/vKjhY9WMv

It looks related to ADL changes for function templates lookup with explicitly-specified template arguments in C++20: https://en.cppreference.com/w/cpp/language/adl.html#Notes
GCC and MSVC both allow unqualified names in template friend declaration friend void f<>(A&);, which is accepted if function template f is declared after class A body but before its instantiation.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Fedor Chelnokov (Fedr)

This program ```c++ template <typename T> class A { friend void f<>(A); };

template <typename T>
void f(A<T>) {}

int main() {
f(A<int>{});
}

is accepted in GCC and MSVC starting from C++20. But Clang keeps complaining:

error: no candidate function template was found for dependent friend function template specialization
3 | friend void f<>(A);
| ^

Online demo: https://gcc.godbolt.org/z/vKjhY9WMv

It looks related to ADL changes for function templates lookup with explicitly-specified template arguments in C++20: https://en.cppreference.com/w/cpp/language/adl.html#Notes 
GCC and MSVC both allow unqualified names in template friend declaration `friend void f<>(A&);`, which is accepted if function template `f` is declared after class `A` body but before its instantiation.

</details>

[shashi1687]

Hi everyone!
I’ve read through the issue and plan to add the required forward declarations in along with a small libc++ regression test to ensure C++20 compliance.
I’d like to take this on and submit a PR to resolve #147177.
Could someone please assign this issue to me?
Thanks!