c++: Fix verify_ctor_sanity ICE [PR96241]

The code added in r10-6437 caused us to create a CONSTRUCTOR when we're
{}-initializing an aggregate.  Then we pass this new CONSTRUCTOR down to
cxx_eval_constant_expression which, if the CONSTRUCTOR isn't TREE_CONSTANT
or reduced_constant_expression_p, calls cxx_eval_bare_aggregate.  In
this case the CONSTRUCTOR wasn't reduced_constant_expression_p because
for r_c_e_p a CONST_DECL isn't good enough so it returns false.  So we
go to cxx_eval_bare_aggregate where we crash, because ctx->ctor wasn't
set up properly.  So my fix is to do so.  Since we're value-initializing,
I'm not setting CONSTRUCTOR_NO_CLEARING.  To avoid keeping a garbage
constructor around, I call free_constructor in case the evaluation did
not use it.

gcc/cp/ChangeLog:

	PR c++/96241
	* constexpr.c (cxx_eval_array_reference): Set up ctx->ctor if we
	are initializing an aggregate.  Call free_constructor on the new
	CONSTRUCTOR if it isn't returned from cxx_eval_constant_expression.

gcc/testsuite/ChangeLog:

	PR c++/96241
	* g++.dg/cpp0x/constexpr-96241.C: New test.
	* g++.dg/cpp1y/constexpr-96241.C: New test.

1 files changed, 9 insertions, 2 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 4707d29..7ebdd30 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3657,15 +3657,22 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
      initializer, it's initialized from {}.  But use build_value_init
      directly for non-aggregates to avoid creating a garbage CONSTRUCTOR.  */
   tree val;
+  constexpr_ctx new_ctx;
   if (CP_AGGREGATE_TYPE_P (elem_type))
     {
       tree empty_ctor = build_constructor (init_list_type_node, NULL);
       val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
+      new_ctx = *ctx;
+      new_ctx.ctor = build_constructor (elem_type, NULL);
+      ctx = &new_ctx;
     }
   else
     val = build_value_init (elem_type, tf_warning_or_error);
-  return cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
-				       overflow_p);
+  t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
+				    overflow_p);
+  if (CP_AGGREGATE_TYPE_P (elem_type) && t != ctx->ctor)
+    free_constructor (ctx->ctor);
+  return t;
 }
 
 /* Subroutine of cxx_eval_constant_expression.