Worker/Wrapper for sum return

GHC version 8.2 introduces Unboxed Sum types. It would be great if the worker/wrapper transformation could make use of this new functionality.

For clarification I would expect a function like this:

maybeEven :: Int -> Maybe Int
maybeEven n = case even n of
  True  -> Just n
  False -> Nothing

to be transformed into (the core equivalent) of

maybeEven :: Int -> Maybe Int
MaybeEven (I# n#) = case workerMaybeEven n# of
  (# | x #)    -> Just x
  (# (# #) | #) -> Nothing

workerMaybeEven :: Int# -> (# (# #) | Int #)
{-# NOINLINE workerMaybeEven #-}
workerMaybeEven n# = case even (I# n#) of
  True  -> (# | I# n# #)
  False -> (# (# #) | #)

Currently the core output for the maybeEven worker is:

Main.$wmaybeEven :: Int# -> Maybe Int
Main.$wmaybeEven
  = \ (ww_s4WH :: Int#) ->
      case remInt# ww_s4WH 2# of {
        __DEFAULT -> GHC.Base.Nothing @ Int;
        0# -> GHC.Base.Just @ Int (GHC.Types.I# ww_s4WH)
      }

Trac metadata

Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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