c++: Fix constexpr vs. omitted aggregate init.

Value-initialization is importantly different from {}-initialization for
this testcase, where the former calls the deleted S constructor and the
latter initializes S happily.

	PR c++/90951
	* constexpr.c (cxx_eval_array_reference): {}-initialize missing
	elements instead of value-initializing them.

1 files changed, 10 insertions, 2 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index c35ec5a..8a02c6e 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3324,8 +3324,16 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
     }
 
   /* If it's within the array bounds but doesn't have an explicit
-     initializer, it's value-initialized.  */
-  tree val = build_value_init (elem_type, tf_warning_or_error);
+     initializer, it's initialized from {}.  But use build_value_init
+     directly for non-aggregates to avoid creating a garbage CONSTRUCTOR.  */
+  tree val;
+  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);
+    }
+  else
+    val = build_value_init (elem_type, tf_warning_or_error);
   return cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
 				       overflow_p);
 }