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+{-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving, StandaloneDeriving, FlexibleInstances #-}
+
+-- Test Trac #2856
+
+module T2856 where
+
+import Data.Ratio
+
+----------------------
+class C a where
+    data D a
+
+instance C Bool where
+    newtype D Bool = DInt Int deriving (Eq, Show, Num)
+
+instance C a => C [a] where
+    newtype D [a] = DList (Ratio a) deriving (Eq, Show, Num)
+
+----------------------
+data family W a
+newtype instance W Bool = WInt Int deriving( Eq, Show )
+newtype instance W [a] = WList (Ratio a) deriving( Eq, Show )
+
+deriving instance Num (W Bool)
+deriving instance (Integral a, Num a) => Num (W [a])
+  -- Integral needed because superclass Eq needs it,
+  -- because of the stupid context on Ratio
+