[Bug c++/107079] [10/11/12/13 Regression] ICE initializing lifetime-extended constexpr variable that stores its this pointer

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107079

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Very interesting.  We're in store_init_value, initializing x with
&TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>
but we must lifetime-extend via extend_ref_init_temps and we get

_ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct
X &) &_ZGR1x_;

Since x was declared constexpr, we do cxx_constant_init (not quiet) and we hit
the code added in r269003:

+  bool preeval = SCALAR_TYPE_P (type) || TREE_CODE (t) == MODIFY_EXPR;
+  if (preeval)
+    {
+      /* Evaluate the value to be stored without knowing what object it will
be
+    stored in, so that any side-effects happen first.  */
+      if (!SCALAR_TYPE_P (type))
+   new_ctx.ctor = new_ctx.object = NULL_TREE;
+      init = cxx_eval_constant_expression (&new_ctx, init, false,
+                      non_constant_p, overflow_p);

while evaluating the INIT_EXPR _ZGR1x_.x = (const struct X *)
&<PLACEHOLDER_EXPR struct X> >>>.  But we have no ctx.ctor or ctx.object here
so lookup_placeholder won't find anything and we ICE on
 7861       /* A placeholder without a referent.  We can get here when
 7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
 7863          just say it's non-constant for now.  */
 7864       gcc_assert (ctx->quiet);

Somehow we should manage that the PLACEHOLDER_EXPR is replaced with _ZGR1x_ I
guess.