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diff --git a/testsuite/tests/typecheck/should_compile/T3955.hs b/testsuite/tests/typecheck/should_compile/T3955.hs
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+++ b/testsuite/tests/typecheck/should_compile/T3955.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
+
+-- Test for Trac #3955
+
+module T3955 where
+
+class (Monad m) => MonadReader r m 
+newtype Reader r a = Reader { runReader :: r -> a }
+
+instance Monad (Reader r) where
+  (>>=)  = error "urk"
+  return = error "urk"
+
+instance MonadReader r (Reader r)
+
+newtype T a x = T (Reader a x)
+    deriving (Monad, MonadReader a)
+
+{-
+[1 of 1] Compiling Main             ( bug.hs, interpreted )
+mkUsageInfo: internal name? a{tv amy}
+Ok, modules loaded: Main.
+-}