PR c++/84785 - ICE with alias template and default targs.

	* pt.c (type_unification_real): Set processing_template_decl if
	saw_undeduced == 1.

From-SVN: r258407

gcc/cp/ChangeLog

 2018-03-09  Jason Merrill  <jason@redhat.com>
 
+	PR c++/84785 - ICE with alias template and default targs.
+	* pt.c (type_unification_real): Set processing_template_decl if
+	saw_undeduced == 1.
+
 	PR c++/84752 - ICE with capture of constexpr array.
 	* call.c (standard_conversion): Set rvaluedness_matches_p on the
 	identity conversion under ck_lvalue.

gcc/cp/pt.c

@@ -19977,7 +19977,13 @@ type_unification_real (tree tparms,
 	  location_t save_loc = input_location;
 	  if (DECL_P (parm))
 	    input_location = DECL_SOURCE_LOCATION (parm);
+
+	  if (saw_undeduced == 1)
+	    ++processing_template_decl;
 	  arg = tsubst_template_arg (arg, full_targs, fcomplain, NULL_TREE);
+	  if (saw_undeduced == 1)
+	    --processing_template_decl;
+
 	  if (arg != error_mark_node && !uses_template_parms (arg))
 	    arg = convert_template_argument (parm, arg, full_targs, complain,
 					     i, NULL_TREE);

gcc/testsuite/g++.dg/cpp0x/alias-decl-63.C

+// PR c++/84785
+// { dg-do compile { target c++11 } }
+
+template <typename> struct A;
+template <bool> struct B;
+template <bool B, typename> using enable_if_t = typename B<B>::type;
+template <long> using type_pack_element = int;
+struct variant {
+  variant() {}
+  template <typename Arg, long I = Arg::type::value,
+            typename = type_pack_element<I>, enable_if_t<A<Arg>::value, int>>
+  variant(Arg &&);
+};
+
+struct S {
+  variant var;
+};
+int main() { S s; }