[GHC] #9981: Potential typechecker regression in GHC 7.10.1RC

[GHC]

#9981: Potential typechecker regression in GHC 7.10.1RC
-------------------------------------+-------------------------------------
              Reporter:  hvr         |             Owner:  simonpj
                  Type:  bug         |            Status:  new
              Priority:  highest     |         Milestone:  7.10.1
             Component:  Compiler    |           Version:  7.10.1-rc1
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  GHC rejects
          Architecture:              |  valid program
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 The following snippet (extracted from hackage:index-core) is accepted by
 GHCs prior to GHC 7.10:

 {{{#!hs
 {-# 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...

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9981>
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