[Bug c++/79398] New: misleading error static constexpr member function called in a constant expression before its definition is complete

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79398

            Bug ID: 79398
           Summary: misleading error static constexpr member function
                    called in a constant expression before its definition
                    is complete
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The error below is a bit confusing: the definition of B::bar() is complete when
it's called.

I think I understand that the reason for the error below is actually that the
function is called before the definition of the class of which it's a member is
complete.  The error should make that clear, although it seems that accepting
it (e.g., as an extension) would make static constexpr member functions quite a
bit more useful.

$ cat y.C && gcc -S -Wall -Wextra -Wpedantic y.C 
struct A {
  static constexpr int foo () { return 1; }
};

struct B: A {
  static constexpr int bar () { return 2; }
  enum E { e = B::foo () };
  enum F { f = B::bar () };
};
y.C:8:23: error: ‘static constexpr int B::bar()’ called in a constant
expression before its definition is complete
   enum F { f = B::bar () };
                ~~~~~~~^~
y.C:8:24: error: enumerator value for ‘f’ is not an integer constant
   enum F { f = B::bar () };
                        ^