GeneralizedNewtypeDeriving sometimes needs RankNTypes

Consider

{-# LANGUAGE RankNTypes #-}

module A where

class C m where
  foo :: (forall b. b -> m b) -> c -> m c

instance C [] where
  foo f c = f c

and

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module B where

import A

newtype MyList a = Mk [a]
  deriving C

When I compile B.hs, I get

    Illegal polymorphic or qualified type: forall b. b -> MyList b
    Perhaps you intended to use RankNTypes or Rank2Types
    In an expression type signature:
      forall (c :: *). (forall (b :: *). b -> MyList b) -> c -> MyList c
    In the expression:
        GHC.Prim.coerce
          (foo :: (forall (b :: *). b -> [] b) -> c -> [] c) ::
          forall (c :: *). (forall (b :: *). b -> MyList b) -> c -> MyList c
    In an equation for ‛foo’:
        foo
          = GHC.Prim.coerce
              (foo :: (forall (b :: *). b -> [] b) -> c -> [] c) ::
              forall (c :: *). (forall (b :: *). b -> MyList b) -> c -> MyList c

I will fix shortly.

Trac metadata

Trac field	Value
Version	7.8.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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