[GHC] #16033: Rank-n typechecking regression between GHC 8.4 and 8.6

[GHC]

#16033: Rank-n typechecking regression between GHC 8.4 and 8.6
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.6.3
  (Type checker)                     |
           Keywords:  RankNTypes     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following program typechecks on GHC 7.0.4 through 8.4.4:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module Bug where

 f :: (forall a. a -> forall b. b -> c) -> ()
 f (_ :: forall a. a -> forall b. b -> z) = ()
 }}}

 However, it does not typecheck on GHC 8.6.3:

 {{{
 $ /opt/ghc/8.6.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:7:4: error:
     • Couldn't match type ‘b0 -> c’ with ‘forall b. b -> z’
       Expected type: a -> forall b. b -> z
         Actual type: a -> b0 -> c
     • When checking that the pattern signature:
           forall a. a -> forall b. b -> z
         fits the type of its context: forall a. a -> forall b. b -> c
       In the pattern: _ :: forall a. a -> forall b. b -> z
       In an equation for ‘f’:
           f (_ :: forall a. a -> forall b. b -> z) = ()
     • Relevant bindings include
         f :: (forall a. a -> forall b. b -> c) -> () (bound at Bug.hs:7:1)
   |
 7 | f (_ :: forall a. a -> forall b. b -> z) = ()
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16033>
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