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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/70248] constexpr initialization with unspecified equality
 expression accepted
Date: Thu, 15 Oct 2020 11:24:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D70248

--- 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 =3D tree_strip_any_location_wrapper (orig_=
op0);
          tree stripped_orig_op1 =3D tree_strip_any_location_wrapper (orig_=
op1);
          if ((TREE_CODE (stripped_orig_op0) =3D=3D STRING_CST
               && !integer_zerop (cp_fully_fold (op1)))
              || (TREE_CODE (stripped_orig_op1) =3D=3D 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) =3D=3D STRING_CST
          || TREE_CODE (orig_op1) =3D=3D 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.=