Possible type-checker regression in GHC 8.0

The following code (extracted from hackage:microlens) type-checks on GHC 7.10 but not on GHC 8.0/8.1:

{-# LANGUAGE RankNTypes, ScopedTypeVariables #-}

module Failing where

import Control.Applicative ( Const(Const, getConst) )
import Data.Functor.Identity ( Identity(Identity) )

type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t

failing :: forall s t a b . Traversal s t a b -> Traversal s t a b -> Traversal s t a b
failing left right afb s = case pins t of
  [] -> right afb s
  _  -> t afb
  where
    -- t :: (a -> f b) -> f t

    -- TYPECHECKS with GHC 7.10, but not with GHC 8.x:
    Bazaar { getBazaar = t } = left sell s

    -- FAILS TO TYPECHECK with GHC 7.10 and GHC 8.x:
    -- t = getBazaar (left sell s)

    sell :: a -> Bazaar a b b
    sell w   = Bazaar ($ w)

    pins :: ((a -> Const [Identity a] b) -> Const [Identity a] t) -> [Identity a]
    pins f   = getConst (f (\ra -> Const [Identity ra]))

newtype Bazaar a b t = Bazaar { getBazaar :: (forall f. Applicative f => (a -> f b) -> f t) }

instance Functor (Bazaar a b) where
  fmap f (Bazaar k) = Bazaar (fmap f . k)

instance Applicative (Bazaar a b) where
  pure a = Bazaar $ \_ -> pure a
  Bazaar mf <*> Bazaar ma = Bazaar $ \afb -> mf afb <*> ma afb

The following error is emitted on GHC 8.0:

failing.hs:13:11: error:
    • Couldn't match type ‘f’ with ‘Const [Identity a]’
      ‘f’ is a rigid type variable bound by
        the type signature for:
          failing :: forall (f :: * -> *). Applicative f => Traversal s t a b -> Traversal s t a b -> (a -> f b) -> s -> f t
        at failing.hs:11:1
      Expected type: a -> Const [Identity a] b
        Actual type: a -> f b
    • In the first argument of ‘t’, namely ‘afb’
      In the expression: t afb
      In a case alternative: _ -> t afb
    • Relevant bindings include
        t :: (a -> Const [Identity a] b) -> Const [Identity a] t (bound at failing.hs:18:26)
        sell :: a -> Bazaar a b b (bound at failing.hs:24:5)
        pins :: ((a -> Const [Identity a] b) -> Const [Identity a] t) -> [Identity a] (bound at failing.hs:27:5)
        afb :: a -> f b (bound at failing.hs:11:20)
        right :: Traversal s t a b (bound at failing.hs:11:14)
        left :: Traversal s t a b (bound at failing.hs:11:9)
        failing :: Traversal s t a b -> Traversal s t a b -> Traversal s t a b (bound at failing.hs:11:1)

I don't understand why Bazaar t = ... vs t = getBazaar ... makes a difference in GHC 7.10 at all. So I'm not sure if this is a regression or actually something that got fixed in GHC 8.0...

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	bgamari, simonpj
Operating system
Architecture