... something like the attached appears to work. Not sure at the moment
if it could be simplified.
Thanks,
Paolo.
////////////////////
Index: cp/cp-tree.h
===================================================================
--- cp/cp-tree.h (revision 206318)
+++ cp/cp-tree.h (working copy)
@@ -1324,7 +1324,12 @@ enum languages { lang_c, lang_cplusplus, lang_java
/* Nonzero iff TYPE is derived from PARENT. Ignores accessibility and
ambiguity issues. */
#define DERIVED_FROM_P(PARENT, TYPE) \
- (lookup_base ((TYPE), (PARENT), ba_any, NULL, tf_none) != NULL_TREE)
+ (((TYPE) && (!TYPE_P (TYPE) \
+ || TYPE_BINFO (complete_type \
+ (TYPE_MAIN_VARIANT (TYPE))))) \
+ ? lookup_base ((TYPE), (PARENT), ba_any, NULL, tf_none) != NULL_TREE \
+ : ((TYPE) && NON_UNION_CLASS_TYPE_P (TYPE) \
+ && same_type_ignoring_top_level_qualifiers_p ((PARENT), (TYPE))))
/* Gives the visibility specification for a class type. */
#define CLASSTYPE_VISIBILITY(TYPE) \
Index: cp/search.c
===================================================================
--- cp/search.c (revision 206318)
+++ cp/search.c (working copy)
@@ -177,8 +177,9 @@ accessible_base_p (tree t, tree base, bool conside
discovered.
If the base is inaccessible, or ambiguous, then error_mark_node is
- returned. If the tf_error bit of COMPLAIN is not set, no error
- is issued. */
+ returned. If the tf_error bit of COMPLAIN is not set, no error is
+ issued. If T cannot be completed, then NULL_TREE is returned even
+ if BASE is the same type. */
tree
lookup_base (tree t, tree base, base_access access,
Index: testsuite/g++.dg/ext/is_base_of_incomplete-2.C
===================================================================
--- testsuite/g++.dg/ext/is_base_of_incomplete-2.C (revision 0)
+++ testsuite/g++.dg/ext/is_base_of_incomplete-2.C (working copy)
@@ -0,0 +1,5 @@
+struct T;
+
+int check1[__is_base_of(T, T) ? 1 : -1];
+int check2[__is_base_of(T, const T) ? 1 : -1];
+int check3[__is_base_of(volatile T, T) ? 1 : -1];
