Allow compatible type synonyms to be the return type of a GADT data constructor.

Please consider adding support for something like:

{-# LANGUAGE GADTs #-}

data Steps s y where
  Yield :: y -> FK s y -> FK s y
  Done  :: Steps s y 

newtype M y s a = M { unM :: (a -> FK s y) -> FK s y }
type FK s y = s -> Steps s y

Trac metadata

Trac field	Value
Version	7.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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