DeriveFunctor fail

{-# LANGUAGE DeriveFunctor, MagicHash, UnboxedTuples #-}

module Lib where

import GHC.Exts

newtype RmLoopsM a = RmLoopsM { runRmLoops :: Int# -> (# Int#, a #) }

Functor instance for this can be derived like this:

instance Functor RmLoopsM where
  fmap f (RmLoopsM m) = RmLoopsM $ \i -> case m i of
                                           (# i', r #) -> (# i', f r #)

DeriveFunctor instead generates something like this:

instance Functor RmLoopsM where
  fmap f_a2Oh (Lib.RmLoopsM a1_a2Oi)
    = RmLoopsM
        ((\ b6_a2Oj b7_a2Ok
            -> (\ b5_a2Ol
                  -> case b5_a2Ol of {
                       ((#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op)
                         -> (#,#)
                              ((\ b2_a2Oq -> b2_a2Oq) a1_a2Om)
                              ((\ b3_a2Or -> b3_a2Or) a2_a2On)
                              ((\ b4_a2Os -> b4_a2Os) a3_a2Oo)
                              (f_a2Oh a4_a2Op) })
                 (b6_a2Oj ((\ b1_a2Ot -> b1_a2Ot) b7_a2Ok)))
           a1_a2Oi)

which fails with

Main.hs:17:25: error:
    • The constructor ‘(#,#)’ should have 2 arguments, but has been given 4
    • In the pattern: (#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op
      In a case alternative:
          ((#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op)
            -> (#,#)
                 ((\ b2_a2Oq -> b2_a2Oq) a1_a2Om)
                 ((\ b3_a2Or -> b3_a2Or) a2_a2On)
                 ((\ b4_a2Os -> b4_a2Os) a3_a2Oo)
                 (f_a2Oh a4_a2Op)
      In the expression:
        case b5_a2Ol of {
          ((#,#) a1_a2Om a2_a2On a3_a2Oo a4_a2Op)
            -> (#,#)
                 ((\ b2_a2Oq -> b2_a2Oq) a1_a2Om)
                 ((\ b3_a2Or -> b3_a2Or) a2_a2On)
                 ((\ b4_a2Os -> b4_a2Os) a3_a2Oo)
                 (f_a2Oh a4_a2Op) }

I think it's supposed to ignore RuntimeRep args during the fold (TcGenDeriv.functorLikeTraverse).

Tried with: HEAD, 8.0.1.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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