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{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ScopedTypeVariables #-}
module T7220a where
class C a b | b -> a
data X = X
data Y = Y
type family TF b
f :: (forall b. (C a b, TF b ~ Y) => b) -> X
-- This type is really ambiguous
-- GHC 7.8 didn't detect that, and accepted the type, but would fail
-- when given g :: <the same type>
-- g x = f x
-- But it would succeed if you said just
-- g = f
-- Now we fail in all ways!
-- But with simple subsumption (#17775) we
-- no longer get an ambiguity check here
f _ = undefined
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