c++: Fix ICE with omitted template args [PR93956].

reshape_init only wants to work on BRACE_ENCLOSED_INITIALIZER_P, i.e. raw
initializer lists, and here was getting a CONSTRUCTOR that had already been
processed for type A<int>.  maybe_aggr_guide should also use that test.

gcc/cp/ChangeLog
2020-03-10  Jason Merrill  <jason@redhat.com>

	PR c++/93956
	* pt.c (maybe_aggr_guide): Check BRACE_ENCLOSED_INITIALIZER_P.

gcc/cp/ChangeLog

 2020-03-10  Jason Merrill  <jason@redhat.com>
 
+	PR c++/93956
+	* pt.c (maybe_aggr_guide): Check BRACE_ENCLOSED_INITIALIZER_P.
+
+2020-03-10  Jason Merrill  <jason@redhat.com>
+
 	PR c++/93922
 	PR c++/94041
 	PR c++/52320

gcc/cp/pt.c

@@ -28182,7 +28182,7 @@ maybe_aggr_guide (tree tmpl, tree init, vec<tree,va_gc> *args)
   tsubst_flags_t complain = tf_none;
 
   tree parms = NULL_TREE;
-  if (TREE_CODE (init) == CONSTRUCTOR)
+  if (BRACE_ENCLOSED_INITIALIZER_P (init))
     {
       init = reshape_init (type, init, complain);
       if (init == error_mark_node)

gcc/testsuite/g++.dg/cpp1z/class-deduction70.C

+// PR c++/93596
+
+template <typename> struct A {};
+template <typename> struct B {};
+template <typename> struct C {
+  void foo () { B a = A<int> { foo }; } // { dg-error "" }
+};