Fix tuple redefinition bug

core/classes/tuple/tuple-tests.factor

@@ -683,3 +683,17 @@ DEFER: error-y
 [ t ] [ \ error-y tuple-class? ] unit-test
 
 [ f ] [ \ error-y generic? ] unit-test
+
+[ ] [
+    "IN: classes.tuple.tests TUPLE: forget-subclass-test ; TUPLE: forget-subclass-test' < forget-subclass-test ;"
+    <string-reader> "forget-subclass-test" parse-stream
+    drop
+] unit-test
+
+[ ] [ "forget-subclass-test'" "classes.tuple.tests" lookup new "bad-object" set ] unit-test
+
+[ ] [
+    "IN: classes.tuple.tests TUPLE: forget-subclass-test a ;"
+    <string-reader> "forget-subclass-test" parse-stream
+    drop
+] unit-test

core/classes/tuple/tuple.factor

@@ -194,13 +194,17 @@ ERROR: bad-superclass class ;
     [ permute-slots ] [ class>> ] bi
     slots>tuple ;
 
+: outdated-tuple? ( tuple assoc -- ? )
+    over tuple? [
+        [ [ layout-of ] dip key? ]
+        [ drop class "forgotten" word-prop not ]
+        2bi and
+    ] [ 2drop f ] if ;
+
 : update-tuples ( -- )
     outdated-tuples get
     dup assoc-empty? [ drop ] [
-        [
-            over tuple?
-            [ >r layout-of r> key? ] [ 2drop f ] if
-        ] curry instances
+        [ outdated-tuple? ] curry instances
         dup [ update-tuple ] map become
     ] if ;