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{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, NoMonoLocalBinds #-}
-- The NoMonoLocalBinds is crucial to making inference fail
-- See #11948 comment:2
{-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}
module T11948 where
type family F b
newtype Foo r = Foo r
type instance F (Foo r) = Foo (F r)
class Bar a b where
bar :: a -> b
instance (Bar a b) => Bar (Foo a) (Foo b)
bug :: forall zq. (Bar (Foo (F zq)) (Foo zq))
=> Foo (F zq) -> Foo zq
bug sk = let x = bar sk :: Foo zq
in x
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