c++: Alias template in pack expansion [PR99445]

In this testcase, iterative_hash_template_arg checks
alias_template_specialization_p to determine whether to treat a type as a
dependent alias, and structural_comptypes checks
dependent_alias_template_spec_p.  Normally that difference isn't a problem
because canonicalizing template arguments strips non-dependent aliases, but
that wasn't happening for the pack expansion.  Fixed thus.

gcc/cp/ChangeLog:

	PR c++/99445
	* tree.c (strip_typedefs): Handle TYPE_PACK_EXPANSION.

gcc/testsuite/ChangeLog:

	PR c++/99445
	* g++.dg/cpp0x/alias-decl-variadic1.C: New test.

gcc/cp/tree.c

@@ -1722,6 +1722,15 @@ strip_typedefs (tree t, bool *remove_attributes, unsigned int flags)
 			     remove_attributes, flags);
       result = finish_underlying_type (type);
       break;
+    case TYPE_PACK_EXPANSION:
+      type = strip_typedefs (PACK_EXPANSION_PATTERN (t),
+			     remove_attributes, flags);
+      if (type != PACK_EXPANSION_PATTERN (t))
+	{
+	  result = copy_node (t);
+	  PACK_EXPANSION_PATTERN (result) = type;
+	}
+      break;
     default:
       break;
     }

gcc/testsuite/g++.dg/cpp0x/alias-decl-variadic1.C

@@ -0,0 +1,14 @@
+// PR c++/99445
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fchecking=2 --param=hash-table-verification-limit=1000" }
+
+template <class> struct implicit_conversions;
+template <class T>
+using implicit_conversions_t = typename implicit_conversions<T>::type;
+template <class...> struct response_type;
+
+template <class Handle, class... Ts>
+using type1 = response_type<implicit_conversions_t<Ts>...>;
+
+template <class Handle, class... Ts>
+using type2 = response_type<typename implicit_conversions<Ts>::type...>;