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diff --git a/testsuite/tests/typecheck/should_compile/tc118.hs b/testsuite/tests/typecheck/should_compile/tc118.hs
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+++ b/testsuite/tests/typecheck/should_compile/tc118.hs
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+{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
+             UndecidableInstances #-}
+
+-- !!! An instance decl with a context containing a free type variable
+-- The interest here is that there's a "b" in the instance decl
+-- context that isn't mentioned in the instance head.  
+-- Hence UndecidableInstances
+
+module ShouldCompile where
+
+class HasConverter a b | a -> b where
+   convert :: a -> b
+ 
+data Foo a = MkFoo a
+
+instance (HasConverter a b,Show b) => Show (Foo a) where
+   show (MkFoo value) = show (convert value)
+