1 files changed, 44 insertions, 0 deletions
diff --git a/testsuite/tests/deriving/should_compile/T4966.hs b/testsuite/tests/deriving/should_compile/T4966.hs
new file mode 100644
index 0000000000..d7328c6ef6
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T4966.hs
@@ -0,0 +1,44 @@
+{-# LANGUAGE DatatypeContexts #-}
+{-# LANGUAGE EmptyDataDecls #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE OverlappingInstances #-}
+
+module HTk.Toolkit.TreeList (getObjectFromTreeList) where
+
+class Eq c => CItem c
+
+-- A bizarre instance decl!
+-- People who use instance decls like this are asking for trouble
+instance GUIObject w => Eq w where
+  w1 == w2 = toGUIObject w1 == toGUIObject w2
+
+data StateEntry a 
+  = StateEntry (TreeListObject a) a -- Comment this 'a' out and it type checks
+  deriving Eq
+
+-- The delicate point about this test is that we want to 
+-- infer a derived instance decl like this:
+--    instance (CItem a, Eq a) => Eq (StateEntry a)
+-- But note the instance decl for (Eq w) for any w!
+-- There's a danger than we'll use that instance decl
+-- to get the derived instance
+--    instance (CItem a, GUIObject a) => Eq (StateEntry a)
+-- And then that doesn't work subsequently
+
+getObjectFromTreeList :: CItem a => StateEntry a -> Bool  
+getObjectFromTreeList state = state == state
+
+data CItem a => TreeListObject a
+
+instance CItem a => Eq (TreeListObject a)
+
+class GUIObject w where
+  toGUIObject     :: w -> GUIOBJECT
+
+
+data GUIOBJECT
+
+instance Eq GUIOBJECT where
+  (==) = undefined
+  (/=) = undefined