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The following snippet (extracted from hackage:index-core) is accepted by GHCs prior to GHC 7.10:

{-# LANGUAGE Rank2Types, TypeOperators #-}

module Foo where

type a :-> b = forall i . a i -> b i

class IFunctor f where
    fmapI :: (a :-> b) -> (f a :-> f b)

class (IFunctor m) => IMonad m where
    returnI ::  a :-> m a
    bindI   :: (a :-> m b) -> (m a :-> m b)

(?>=) :: (IMonad m) => m a i -> (a :-> m b) -> m b i
(?>=) = flip bindI

$ ghci-7.8.4 -Wall Foo.hs 
GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Control.IMonad.Core ( Foo.hs, interpreted )
Ok, modules loaded: Control.IMonad.Core.
λ:2> :browse
type (:->) (a :: * -> *) (b :: * -> *) = forall i. a i -> b i
class IFunctor (f :: (* -> *) -> * -> *) where
  fmapI :: (a :-> b) -> f a :-> f b
class IFunctor m => IMonad (m :: (* -> *) -> * -> *) where
  returnI :: a i -> m a i
  bindI :: (a :-> m b) -> m a :-> m b
(?>=) :: IMonad m => m a i -> (a :-> m b) -> m b i

vs.

$ ghci-7.10.0.20141227 -Wall Foo.hs 
GHCi, version 7.10.0.20141227: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Control.IMonad.Core ( Foo.hs, interpreted )

Foo.hs:15:14:
    Couldn't match type ‘a i0 -> m b i0’ with ‘forall i1. a i1 -> m b i1’
    Expected type: (a i0 -> m b i0) -> m a i -> m b i
      Actual type: a :-> m b -> m a i -> m b i
    Relevant bindings include (?>=) :: m a i -> a :-> m b -> m b i (bound at Foo.hs:15:1)
    In the first argument of ‘flip’, namely ‘bindI’
    In the expression: flip bindI
Failed, modules loaded: none.
λ:2> 

I'm not sure though whether that module was valid to begin with...

Trac metadata

Trac field	Value
Version	7.10.1-rc1
Type	Bug
TypeOfFailure	ValidProgramRejected
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
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