[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
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.6.3
  checker)                           |
      Resolution:                    |             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:                    |
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Comment (by RyanGlScott):

 This bug also affects rank-//n// //contexts// as well, as the following
 typechecks in GHC 8.4.4 and earlier, but not in GHC 8.6.3:

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

 f    :: (forall a. a -> Show a => c) -> ()
 f (x ::  forall a. a -> Show a => c)  = ()
 }}}
 {{{
 $ /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 ‘c1’ with ‘Show a => c’
       ‘c1’ is a rigid type variable bound by
         the type signature for:
           f :: forall c1. (forall a. a -> Show a => c1) -> ()
         at Bug.hs:6:1-42
       Expected type: a -> Show a => c
         Actual type: a -> c1
     • When checking that the pattern signature:
           forall a. a -> Show a => c
         fits the type of its context: forall a. a -> Show a => c1
       In the pattern: x :: forall a. a -> Show a => c
       In an equation for ‘f’: f (x :: forall a. a -> Show a => c) = ()
     • Relevant bindings include
         f :: (forall a. a -> Show a => c1) -> () (bound at Bug.hs:7:1)
   |
 7 | f (x ::  forall a. a -> Show a => c)  = ()
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16033#comment:3>
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