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{-# LANGUAGE
FlexibleInstances,
UndecidableInstances
#-}
module Bug where
class Default a
class Sat a
instance Default a => Sat a
class Sat a => Data a where
dataTypeOf :: a -> a
defaultDefaultValue :: Data a => a
defaultDefaultValue = res
where
res = dataTypeOf res
-- GHC does not infer the principal type for res,
-- inferring (Default a, Data a) => a instead of Data a => a.
-- See Note [Inferring principal types] in Ghc.Tc.Solver
--
-- This used to be fine, as "Default a" was a Derived constraint
-- that could be dropped. Without Deriveds, we instead get a
-- Wanted constraint, which can't be dropped.
-- This means that this program no longer compiles.
-- (Note that a type signature on "res" allows the program to
-- compile again.)
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