PR c++/84839 - ICE with decltype of parameter pack.

	* pt.c (tsubst_pack_expansion): Set cp_unevaluated_operand while
	instantiating dummy parms.

From-SVN: r258500

gcc/cp/ChangeLog

 2018-03-13  Jason Merrill  <jason@redhat.com>
 
+	PR c++/84839 - ICE with decltype of parameter pack.
+	* pt.c (tsubst_pack_expansion): Set cp_unevaluated_operand while
+	instantiating dummy parms.
+
 	* parser.c (cp_parser_simple_type_specifier): Pedwarn about auto
 	parameter even without -Wpedantic.
 

gcc/cp/pt.c

@@ -11717,7 +11717,9 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	    {
 	      /* This parameter pack was used in an unevaluated context.  Just
 		 make a dummy decl, since it's only used for its type.  */
+	      ++cp_unevaluated_operand;
 	      arg_pack = tsubst_decl (parm_pack, args, complain);
+	      --cp_unevaluated_operand;
 	      if (arg_pack && DECL_PACK_P (arg_pack))
 		/* Partial instantiation of the parm_pack, we can't build
 		   up an argument pack yet.  */

gcc/testsuite/g++.dg/cpp0x/variadic-nested2.C

+// PR c++/84839
+// { dg-do compile { target c++11 } }
+
+template<typename... T>
+struct S {
+    using fptr = void(*)(T... x, decltype(x)... y);
+};
+
+using F = S<int>::fptr;