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I don't have the latest version of GHC, trying to derive Functor A and Foldable A is fine but when I derive Traversable A in the attachment Error.hs:

{-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, ImpredicativeTypes #-}

import Data.Functor     (Functor)
import Data.Foldable    (Foldable)
import Data.Traversable (Traversable)

data A a = A
  deriving (Functor, Foldable, Traversable)

GHC barks at me (verbose log attached):

/tmp/Error.hs:8:32: error:
    • Couldn't match type ‘forall a1. A a1’ with ‘A b’
      Expected type: f (A b)
        Actual type: f (forall a. A a)
    • In the expression: pure A
      In an equation for ‘traverse’: traverse f A = pure A
      When typechecking the code for ‘traverse’
        in a derived instance for ‘Traversable A’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘Traversable A’
    • Relevant bindings include
        f :: a -> f b (bound at /tmp/Error.hs:8:32)
        traverse :: (a -> f b) -> A a -> f (A b)
          (bound at /tmp/Error.hs:8:32)

With -ddump-deriv we get this (unqualified) instance:

  instance Traversable A where
    traverse f_a2Le A = pure A

which by itself causes the same problem in the attachment Error2.hs:

{-# LANGUAGE DeriveFunctor, DeriveFoldable, ImpredicativeTypes #-}

import Data.Functor     (Functor)
import Data.Foldable    (Foldable)
import Data.Traversable (Traversable)

data A a = A
  deriving (Functor, Foldable)

instance Traversable A where
  traverse f A = pure A

Works fine in GHC-7.10.2 and GHC-7.10.0.20150316 and GHC-7.4 (with some additional imports), is this an ImpredicativeTypes regression?

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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