[PR c++/84593] ice on braced init with uninit ref field

If an initializer expr is to be NULL in a ctor initializer list, we
ICE in picflag_from_initializer and elsewhere.

If we're missing an initializer for a reference field, we report the
error, but then build a zero initializer to avoid the ICE.

for  gcc/cp/ChangeLog

	PR c++/84593
	* init.c (build_zero_init_1): Zero-initialize references.

for  gcc/testsuite/ChangeLog

	PR c++/84593
	* g++.dg/cpp1y/pr84593.C: New.

From-SVN: r258270

gcc/cp/ChangeLog

 2018-03-06  Alexandre Oliva <aoliva@redhat.com>
 
+	PR c++/84593
+	* init.c (build_zero_init_1): Zero-initialize references.
+
 	PR c++/84492
 	* semantics.c (finish_stmt_expr_expr): Reject unresolved
 	overloads used as stmt expr values.

gcc/cp/init.c

@@ -284,7 +284,10 @@ build_zero_init_1 (tree type, tree nelts, bool static_storage_p,
   else if (VECTOR_TYPE_P (type))
     init = build_zero_cst (type);
   else
+    {
       gcc_assert (TREE_CODE (type) == REFERENCE_TYPE);
+      init = build_zero_cst (type);
+    }
 
   /* In all cases, the initializer is a constant.  */
   if (init)

gcc/testsuite/ChangeLog

 2018-03-06  Alexandre Oliva <aoliva@redhat.com>
 
+	PR c++/84593
+	* g++.dg/cpp1y/pr84593.C: New.
+
 	PR c++/84492
 	* g++.dg/pr84492.C: New.
 

gcc/testsuite/g++.dg/cpp1y/pr84593.C

+// PR c++/84593
+// { dg-do compile { target c++14 } }
+
+struct a {
+  int x;
+  int c = 0;
+  int &b;
+} c = {}; // { dg-error "uninitialized reference" }