diff --git a/ChangeLog b/ChangeLog
index b882383485..6c4dc09a04 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+Mon Oct  5 00:09:57 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* vm_method.c (basic_obj_respond_to): should not call
+	  #respond_to_missing? for not implemented methods.
+	  [ruby-core:25909]
+
+	* vm_method.c (rb_method_boundp): returns exceptional value 2 for
+	  not-implemented methods when called from #respond_to? (specifies
+	  by new contant NOEX_RESPONDS).
+
+	* method.h (enum): new constant NOEX_RESPONDS added.
+
 Sun Oct  4 22:16:29 2009  Takeyuki FUJIOKA  <xibbar@ruby-lang.org>
 
 	* lib/cgi/cookie.rb: add default value to @@accept_charset
diff --git a/method.h b/method.h
index 11fd6bfef8..177d7bc677 100644
--- a/method.h
+++ b/method.h
@@ -21,7 +21,8 @@ typedef enum {
     NOEX_UNDEF     = NOEX_NOSUPER,
     NOEX_MODFUNC   = 0x12,
     NOEX_SUPER     = 0x20,
-    NOEX_VCALL     = 0x40
+    NOEX_VCALL     = 0x40,
+    NOEX_RESPONDS  = 0x80
 } rb_method_flag_t;
 
 #define NOEX_SAFE(n) ((int)((n) >> 8) & 0x0F)
diff --git a/vm_method.c b/vm_method.c
index ec381c40d7..a31fb8b5ac 100644
--- a/vm_method.c
+++ b/vm_method.c
@@ -515,12 +515,14 @@ rb_method_boundp(VALUE klass, ID id, int ex)
 	if (ex && (me->flag & NOEX_PRIVATE)) {
 	    return FALSE;
 	}
-	if (!me->def || me->def->type == VM_METHOD_TYPE_NOTIMPLEMENTED) {
-	    return FALSE;
+	if (!me->def) return 0;
+	if (me->def->type == VM_METHOD_TYPE_NOTIMPLEMENTED) {
+	    if (ex & NOEX_RESPONDS) return 2;
+	    return 0;
 	}
-	return TRUE;
+	return 1;
     }
-    return FALSE;
+    return 0;
 }
 
 void
@@ -1151,13 +1153,14 @@ basic_obj_respond_to(VALUE obj, ID id, int pub)
 {
     VALUE klass = CLASS_OF(obj);
 
-    if (!rb_method_boundp(klass, id, pub)) {
-	if (!rb_method_basic_definition_p(klass, respond_to_missing)) {
-	    return RTEST(rb_funcall(obj, respond_to_missing, pub ? 1 : 2, ID2SYM(id), Qtrue));
-	}
+    switch (rb_method_boundp(klass, id, pub|NOEX_RESPONDS)) {
+      case 2:
 	return FALSE;
+      case 0:
+	return RTEST(rb_funcall(obj, respond_to_missing, pub ? 1 : 2, ID2SYM(id), Qtrue));
+      default:
+	return TRUE;
     }
-    return TRUE;
 }
 
 int