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{-# LANGUAGE FunctionalDependencies, FlexibleInstances, UndecidableInstances,
ScopedTypeVariables, TypeFamilies, TypeApplications, NoPolyKinds,
FlexibleContexts, AllowAmbiguousTypes #-}
module T18851b where
-- NB: -XNoPolyKinds is important. Otherwise, we get IsInt :: forall k. k -> Constraint,
-- but its instance specializes k to Type. The [W] IsInt int doesn't match the instance
-- then, and so we get no int ~ Int equality.
class C a b | a -> b
instance C Int b => C Int b
class IsInt int
instance int ~ Int => IsInt int
data A
instance Show A where
show _ = "A"
data B
instance Show B where
show _ = "B"
f :: forall a b c int. (Show a, Show b, Show c, C int a, C int b, C int c, IsInt int) => String
f = show (undefined :: c)
g = f @A @B
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