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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/100111] [10/11 Regression] `-fno-elide-constructors` with
 `constexpr` ctors causes ICE in GCC 10.3
Date: Fri, 16 Apr 2021 09:04:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100111

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The ICE is during ((struct j *) this)->e =3D 0 evaluation and the problem s=
eems
to be that *valp, which is CONSTRUCTOR for the a variable initializer, is
{.h=3D{}}
where the type of the inner {} is b rather than j.
That invalid CONSTRUCTOR elt seems to have been inserted when
cxx_eval_store_expression
*(struct b *) this =3D *(const struct b &) (const struct b *) &k;
I wonder if the bug isn't in:
  /* Don't share a CONSTRUCTOR that might be changed later.  */
  init =3D unshare_constructor (init);

  if (*valp && TREE_CODE (*valp) =3D=3D CONSTRUCTOR
      && TREE_CODE (init) =3D=3D CONSTRUCTOR)
    {
      /* An outer ctx->ctor might be pointing to *valp, so replace
         its contents.  */
      if (!same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (init),
                                                      TREE_TYPE (*valp)))
        {
          /* For initialization of an empty base, the original target will =
be
           *(base*)this, evaluation of which resolves to the object
           argument, which has the derived type rather than the base type. =
 In
           this situation, just evaluate the initializer and return, since
           there's no actual data to store.  */
          gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval);
          return init;
        }
      CONSTRUCTOR_ELTS (*valp) =3D CONSTRUCTOR_ELTS (init);
      TREE_CONSTANT (*valp) =3D TREE_CONSTANT (init);
      TREE_SIDE_EFFECTS (*valp) =3D TREE_SIDE_EFFECTS (init);
      CONSTRUCTOR_NO_CLEARING (*valp)
        =3D CONSTRUCTOR_NO_CLEARING (init);
    }
  else
    *valp =3D init;

where we do that if (!same_type_ignoring... check and early out only if *va=
lp,
but in this case *valp is NULL and so we do the *valp =3D init; , except th=
at
it is still empty class store with !lval.  While TREE_TYPE (*valp) can't be
checked obviously, type can.=