Index: testsuite/g++.dg/ext/is_base_of_incomplete.C
===================================================================
--- testsuite/g++.dg/ext/is_base_of_incomplete.C	(revision 0)
+++ testsuite/g++.dg/ext/is_base_of_incomplete.C	(revision 0)
@@ -0,0 +1,7 @@
+template <typename T>
+struct non_instantiable
+{
+  typedef typename T::THIS_TYPE_CANNOT_BE_INSTANTIATED type;
+};
+
+int check[__is_base_of(non_instantiable<int>, void) ? -1 : 1];
Index: cp/semantics.c
===================================================================
--- cp/semantics.c	(revision 179997)
+++ cp/semantics.c	(working copy)
@@ -5276,10 +5276,6 @@ finish_trait_expr (cp_trait_kind kind, tree type1,
       return trait_expr;
     }
 
-  complete_type (type1);
-  if (type2)
-    complete_type (type2);
-
   switch (kind)
     {
     case CPTK_HAS_NOTHROW_ASSIGN:
@@ -5297,6 +5293,7 @@ finish_trait_expr (cp_trait_kind kind, tree type1,
     case CPTK_IS_POLYMORPHIC:
     case CPTK_IS_STD_LAYOUT:
     case CPTK_IS_TRIVIAL:
+      complete_type (type1);
       if (!check_trait_type (type1))
 	{
 	  error ("incomplete type %qT not allowed", type1);
@@ -5305,6 +5302,7 @@ finish_trait_expr (cp_trait_kind kind, tree type1,
       break;
 
     case CPTK_IS_BASE_OF:
+      complete_type (type2);
       if (NON_UNION_CLASS_TYPE_P (type1) && NON_UNION_CLASS_TYPE_P (type2)
 	  && !same_type_ignoring_top_level_qualifiers_p (type1, type2)
 	  && !COMPLETE_TYPE_P (type2))