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{-# LANGUAGE FunctionalDependencies, FlexibleInstances, UndecidableInstances,
ScopedTypeVariables, TypeFamilies, TypeApplications,
FlexibleContexts, AllowAmbiguousTypes, ExtendedDefaultRules #-}
module T18851 where
default (Int)
type family C_FD a
class C_FD a ~ b => C a b
type instance C_FD Int = Bool -- just for Show (C_FD Int)
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 c, Num int
, C int a, C int b, C int c
-- , c ~ C_FD int -- add this to get rid of ambiguity error
)
=> String
f = show (undefined :: c)
-- blows up at run time once type checks
g :: String
g = f @A @B
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