[Bug c++/107439] use of static member function in requires-expression depends on declaration order

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107439

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
> This seems inconsistent, as member functions are normally expected to be usable anywhere within the class definition.

IIUC associated constraints are part of the function signature and thus aren't
late parsed like the function body is, so later-declared members aren't
generally usable in a constraint.

Interestingly, if we add a dummy argument to 'check' then we accept the call
(and treat it as an ADL-enabled call to an unknown function template where
unqualified lookup found nothing):

struct A
{
  template<typename T> requires (check<T>(0))
  auto func(T) { }

  template<typename T>
  static consteval bool check(0) { return true; }
};

But if we then try to actually use func, its constraint will always fail due to
'check' not being visible at the point of use (since associated constraints
aren't late-parsed):

int main() {
  A a;
  a.func(0); // error: ‘check’ was not declared in this scope, and no
declarations were found by argument-dependent lookup at the point of
instantiation
}

This behavior (for the modified testcase) is correct AFAICT (Clang behaves the
same).