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.

gcc/cp/ChangeLog

 2020-02-04  Jason Merrill  <jason@redhat.com>
 
+	PR c++/90951
+	* constexpr.c (cxx_eval_array_reference): {}-initialize missing
+	elements instead of value-initializing them.
+
 	PR c++/86917
 	* init.c (perform_member_init): Simplify.
 	* constexpr.c (cx_check_missing_mem_inits): Allow uninitialized

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);
 }

gcc/testsuite/g++.dg/cpp0x/constexpr-array24.C

+// PR c++/90951
+// { dg-do compile { target c++11 } }
+
+#define assert(expr) static_assert (expr, #expr)
+
+struct S { const char a[2]; };
+
+constexpr struct S a[1][1][1] = { };
+
+assert ('\0' == *a[0][0][0].a);