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The following implementation of Maybe using UnboxedSums results in Non-exhaustive patterns in case:

(Failure.hs file)

{-# LANGUAGE UnboxedSums #-}
{-# LANGUAGE PatternSynonyms #-}

type Maybe' t = (# t | () #)

pattern Just' :: a -> Maybe' a
pattern Just' x = (# x | #)

pattern Nothing' :: Maybe' a
pattern Nothing' = (# | () #)

foo x = case x of
  Nothing' -> putStrLn "nothing"
  Just' _ -> putStrLn "just"

main = do
  putStrLn "Nothing'"
  foo Nothing'

  putStrLn "Just'"
  foo (Just' "hello")

When executed, it prints:

Nothing'
nothing
Just'
Failure: Failure.hs:10:20-29: Non-exhaustive patterns in case

Compiled with ghc Failure.hs.

Please note that by removing the patterns, and writting foo as following works as expected:

foo x = case x of
  (# | () #) -> putStrLn "nothing"
  (# _ | #) -> putStrLn "just"

Trac metadata

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