[Bug c++/98122] [10/11 Regression] Accessing union member through pointer-to-member is not a constant expression

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98122

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
--- gcc/cp/constexpr.c.jj       2020-12-03 15:43:00.491620290 +0100
+++ gcc/cp/constexpr.c  2020-12-04 13:02:06.874418746 +0100
@@ -4679,7 +4679,8 @@ cxx_fold_indirect_ref_1 (location_t loc,
        }
     }
   /* ((foo *)&struct_with_foo_field)[x] => COMPONENT_REF */
-  else if (TREE_CODE (optype) == RECORD_TYPE)
+  else if (TREE_CODE (optype) == RECORD_TYPE
+          || TREE_CODE (optype) == UNION_TYPE)
     {
       for (tree field = TYPE_FIELDS (optype);
           field; field = DECL_CHAIN (field))
would fix this, but wonder if for unions we don't need to take into account the
currently active member.

In:
union U { int a; char b; };

constexpr bool
foo ()
{
  U f { .b = 42 };
  constexpr auto m = &U::a;
  return (f.*m) == 42;
}

static_assert (foo (), "");
we even with the patch properly reject it:
pr98122-2.C:11:20: error: non-constant condition for static assertion
   11 | static_assert (foo (), "");
      |                ~~~~^~
pr98122-2.C:11:20:   in ‘constexpr’ expansion of ‘foo()’
pr98122-2.C:11:20: error: accessing ‘U::a’ member instead of initialized ‘U::b’
member in constant expression

But:
union U { int a; int b; };

constexpr bool
foo ()
{
  U f { .b = 42 };
  constexpr auto m = &U::b;
  return (f.*m) == 42;
}

static_assert (foo (), "");
is rejected too and I think it should be accepted.
So I guess for UNION_TYPEs we need to first try the active member and only then
fall back to what the code would do after the above patch.