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{-# LANGUAGE TypeFamilies, TypeFamilyDependencies #-}
module T13651 where
type family F r s = f | f -> r s
type instance F (Bar h (Foo r)) (Bar h (Foo s)) = Bar h (Bar r s)
data Bar s b
data Foo a
foo :: (F cr cu ~ Bar h (Bar r u),
F cu cs ~ Bar (Foo h) (Bar u s))
=> Bar h (Bar r u) -> Bar (Foo h) (Bar u s) -> Foo (cr -> cs)
foo = undefined
{- Typechecking this program used to /just/ succeed in GHC 8.2,
(see Trac #14745 for why), but doesn't in 8.4.
[G] F cr cu ~ Bar h (Bar r u),
F cu cs ~ Bar (Foo h) (Bar u s))
[W] F cr cu0 ~ Bar h (Bar r u)
--> (top-level fundeps) cr ~ Bar h (Foo r)
cu0 ~ Bar h (Foo u)
(local fundeps) cu ~ cu0
[W] F cu0 cs ~ Bar (Foo h) (Bar u s)
--> (top-level fundeps) cu0 ~ Bar (Foo h) (Foo u)
cs ~ Bar (Foo h) (Foo s)
(local fundeps) cu0 ~ cu
[W] F cr (Bar (Foo h) (Fo u)) ~ Bar h (Bar r u)
-}
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