[GHC] #14618: Higher rank typechecking is broken

[GHC]

#14618: Higher rank typechecking is broken
-------------------------------------+-------------------------------------
           Reporter:  mniip          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The oddity was noticed when someone asked the type of `lens` from
 lambdabot and it replied with
 {{{#!hs
 1514165211 [04:26:51] <lambdabot> Functor f => (s1 -> a1) -> (s1 -> b1 ->
 t1) -> (a2 -> f b2) -> s2 -> f t2
 }}}

 This disagrees with `lens`'s type per definition (way too many foralls and
 unconstrained type variables), and if handled carefully it could be used
 to implement `unsafeCoerce` within SafeHaskell.

 The minimalest example I could come up with:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}

 safeCoerce :: a -> b
 safeCoerce = f'
     where
         f :: d -> forall c. d
         f x = x

         f' = f
 }}}

 This compiles and typechecks on GHC 8.2.2, 8.2.1 (courtesy of hydraz) and
 HEAD.
 But not on 8.0.2 (hydraz) or 7.10.3.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14618>
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