[Clang] Friend function reported to be undeclared unless one of the arguments is the class type

[ShawnZhong]

In the example below, foo is reported to be undeclared unless one of the arguments is the class type.

struct S {
  friend void foo(){};
  friend void foo(void*){};
  friend void foo(void*, void*){};
};

void test() {
  foo();  // use of undeclared identifier 'foo'

  foo(nullptr);      // use of undeclared identifier 'foo'
  foo((S*)nullptr);  // no error

  foo(nullptr, nullptr);          // use of undeclared identifier 'foo'
  foo((S*)nullptr, nullptr);      // no error
  foo(nullptr, (S*)nullptr);      // no error
  foo((S*)nullptr, (S*)nullptr);  // no error
}

I think that in all the cases, foo should be declared (and also defined), so the program should be valid.
https://eel.is/c++draft/class.friend#example-6 provides an example of defining friend function within the class.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

I think if I read class.friend p6 and class.friend p11 it sure seems that way.

but I think the key here is decl.meaning.general p2.1 which says:

If the declaration is a friend declaration:

• (2.1) The declarator does not bind a name.

clang/gcc/MSVS all seem to agree here: https://godbolt.org/z/fT9fse7K5

I do feel like the wording could use some clarification and an example that demonstrates this case seems warranted.

The old wording which we can find in DR 1477 also is consistent with the behavior we see.

CC @hubert-reinterpretcast for conformance check-in