1 files changed, 47 insertions, 0 deletions
diff --git a/testsuite/tests/gadt/gadt16.hs b/testsuite/tests/gadt/gadt16.hs
new file mode 100644
index 0000000000..133c833903
--- /dev/null
+++ b/testsuite/tests/gadt/gadt16.hs
@@ -0,0 +1,47 @@
+{-# LANGUAGE GADTs #-}
+
+{- This code, courtesy of Markus Lauer (markus.lauer-2006@lauerit.de)
+   was rejected by the sophisticated wobbly-type impl in 6.4.1, and
+   with a terrible error message:
+     Sample.hs:22:40:
+       Couldn't match `MayFail' against `MayFail'
+         Expected type: Result s b
+         Inferred type: Result MayFail a
+       In the first argument of `Return', namely `Fail'
+       In a case alternative: Fail -> return Fail
+
+   Strangely it is accepted by the simplified impl in GHC 6.5. -}
+
+module Sample where
+
+data Safe
+data MayFail
+
+data Result s a where
+   Ok   :: a   -> Result s       a
+   Fail ::        Result MayFail a
+
+newtype M s a = M { unM :: IO (Result s a) }
+
+instance Monad (M s) where  
+
+  return x = M (return (Ok x))
+  
+  {- this one gives a type error in 6.4.1 -}
+  M m >>= k = M (do res <- m
+                    case res of 
+                         Ok x -> unM (k x)
+                         Fail -> return Fail
+                     )                    
+
+  {- while this one works -}  
+  -- M m >>= k = M (f m (unM . k))
+  --   where 
+  --     f :: IO (Result s a) -> (a -> IO (Result s b)) -> IO (Result s b)
+  --     f m k = do res <- m
+  --                case res of
+  --                     Ok x -> k x
+  --                     Fail -> return Fail
+ 
+  
+