deriving, wrong code: newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving stock Functor

This fails (deriving stock Functor) (data type comes from Applicative Archery)

$ ./ghc-stage2 --interactive -ignore-dot-ghci
GHCi, version 8.7.20181230: https://www.haskell.org/ghc/  :? for help
Prelude> :set prompt "> "
> :set -XRankNTypes -XDeriveFunctor -ddump-deriv
> :set -dsuppress-idinfo -dsuppress-coercions -dsuppress-type-applications -dsuppress-uniques -dsuppress-module-prefixes
> newtype T cat a = MkT ((forall xx. cat xx xx) -> a) deriving Functor

==================== Derived instances ====================
Derived class instances:
  instance Functor (T cat) where
    fmap f (MkT a1) = MkT ((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1)
    (<$) z (MkT a1)
      = MkT ((\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))) a1)


Derived type family instances:



<interactive>:4:62: error:
    • Couldn't match type ‘forall xx. cat xx xx’ with ‘cat xx0 xx0’
      Expected type: cat xx0 xx0 -> a
        Actual type: (forall xx. cat xx xx) -> a
    • In the first argument of ‘\ b2 b3
                                  -> f (b2 ((\ b1 -> b1) b3))’, namely
        ‘a1’
      In the first argument of ‘MkT’, namely
        ‘((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1)’
      In the expression: MkT ((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1)
      When typechecking the code for ‘fmap’
        in a derived instance for ‘Functor (T cat)’:
        To see the code I am typechecking, use -ddump-deriv
    • Relevant bindings include
        a1 :: (forall xx. cat xx xx) -> a (bound at <interactive>:4:62)
        fmap :: (a -> b) -> T cat a -> T cat b
          (bound at <interactive>:4:62)

<interactive>:4:62: error:
    • Couldn't match type ‘forall xx. cat xx xx’ with ‘cat xx1 xx1’
      Expected type: cat xx1 xx1 -> b
        Actual type: (forall xx. cat xx xx) -> b
    • In the first argument of ‘\ b3 b4
                                  -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))’, namely
        ‘a1’
      In the first argument of ‘MkT’, namely
        ‘((\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))) a1)’
      In the expression:
        MkT ((\ b3 b4 -> (\ b2 -> z) (b3 ((\ b1 -> b1) b4))) a1)
      When typechecking the code for ‘<$’
        in a derived instance for ‘Functor (T cat)’:
        To see the code I am typechecking, use -ddump-deriv
    • Relevant bindings include
        a1 :: (forall xx. cat xx xx) -> b (bound at <interactive>:4:62)
        (<$) :: a -> T cat b -> T cat a (bound at <interactive>:4:62)
>

Generates odd code

fmap .. = MkT ((\ b2 b3 -> f (b2 ((\ b1 -> b1) b3))) a1)

It works fine (in HEAD) to deriving newtype Functor (it didn't in 8.2 where I checked)

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture