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

[jwjagersma at gmail dot com]

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

            Bug ID: 107439
           Summary: use of static member function in requires-expression
                    depends on declaration order
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jwjagersma at gmail dot com
  Target Milestone: ---

The following example fails to compile, as g++ complains that a declaration of
'check' is not available.  Reordering it so that 'check' is declared before
its use in the requires-expression makes it work.  This seems inconsistent, as
member functions are normally expected to be usable anywhere within the class
definition.

  $ cat requires-memfn.cpp
  struct A
  {
    template<typename T> requires (check<T>())
    auto func(T) { }

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

  $ g++ -std=c++20 requires-memfn.cpp
  requires-memfn.cpp:3:34: error: there are no arguments to ‘check’ that depend
on a template parameter, so a declaration of ‘check’
   must be available [-fpermissive]
      3 |   template<typename T> requires (check<T>())
        |                                  ^~~~~~~~
  requires-memfn.cpp:3:34: note: (if you use ‘-fpermissive’, G++ will accept
your code, but allowing the use of an undeclared name is deprecated)