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...