1 files changed, 26 insertions, 0 deletions
diff --git a/testsuite/tests/indexed-types/should_fail/T4179.hs b/testsuite/tests/indexed-types/should_fail/T4179.hs
new file mode 100644
index 0000000000..ee54100ccc
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+++ b/testsuite/tests/indexed-types/should_fail/T4179.hs
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+{-# LANGUAGE TypeFamilies  #-}
+
+module T4179 where
+
+class DoC a where
+     type A2 a
+     type A3 a
+     op :: a -> A2 a -> A3 a
+
+data Con x = InCon (x (Con x))
+type FCon x = x (Con x)
+
+-- should have been changed to this, which works
+-- foldDoC :: Functor f => (f a -> a) -> A2 (FCon f) -> Con f -> a
+-- foldDoC f i (InCon t) = f (fmap (foldDoC f i) t)
+-- this original version causes GHC to hang 
+foldDoC :: Functor f => (f a -> a) -> Con f -> a 
+foldDoC f (InCon t) = f (fmap (foldDoC f) t)
+
+doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) 
+doCon (InCon x) = op x
+
+-- Note that if this is commented out then there's no hang: 
+-- presumably because GHC doesn't have to perform type deduction for foldDoC.
+fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) 
+fCon = foldDoC op