[Bug c++/69681] New: C/C++ FEs do not consider comparisons of distinct function pointers to be constant expressions

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69681

            Bug ID: 69681
           Summary: C/C++ FEs do not consider comparisons of distinct
                    function pointers to be constant expressions
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

The following test case fails to compile with both the C and C++ FEs:

  void foo () { }
  void bar () { }

  int x[(int)(&foo != &bar)];

The comparison "&foo != &bar" ought to be folded to 1 since both foo and bar
distinct, defined functions at the point when the comparison is made.

It is less obvious whether the comparison should be folded if foo and bar are
not yet defined:

   void foo ();
   void bar ();
   int x[(int)(&foo != &bar)];

Because at this point one can add another declaration of foo and bar that give
them the "weak" attribute, which would allow for &foo and &bar to be NULL
pointers, thus actually making the result of the comparison indeterminate at
compile time.  If such code is allowed, the subsequent declarations of foo and
bar must not declare these functions as weak, alias, etc, like in PR c++/61825.

This issue also shows up when comparing and pointers to C++ member functions,
and in C++ constexpr contexts.