[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In gcc/cp/typeck.c:cp_build_binary_op this warning should be an error during
constant evaluation for EQ_EXPR and NE_EXPR:

      if (complain & tf_warning)
        {
          tree stripped_orig_op0 = tree_strip_any_location_wrapper (orig_op0);
          tree stripped_orig_op1 = tree_strip_any_location_wrapper (orig_op1);
          if ((TREE_CODE (stripped_orig_op0) == STRING_CST
               && !integer_zerop (cp_fully_fold (op1)))
              || (TREE_CODE (stripped_orig_op1) == STRING_CST
                  && !integer_zerop (cp_fully_fold (op0))))
            warning_at (location, OPT_Waddress,
                        "comparison with string literal results in "
                        "unspecified behavior");
        }

And this one for rel ops and spaceships:

      if (TREE_CODE (orig_op0) == STRING_CST
          || TREE_CODE (orig_op1) == STRING_CST)
        {
          if (complain & tf_warning)
            warning_at (location, OPT_Waddress,
                        "comparison with string literal results "
                        "in unspecified behavior");
        }

That only handles the string literals cases though, we also need to reject
pointers to distinct objects at the same places.