[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 (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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Description changed by RyanGlScott:

Old description:

> 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) = ()
>   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> }}}

New description:

 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 -> c)  = ()
 }}}

 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 -> c1’ with ‘forall b. b -> c’
       Expected type: a -> forall b. b -> c
         Actual type: a -> b0 -> c1
     • When checking that the pattern signature:
           forall a. a -> forall b. b -> c
         fits the type of its context: forall a. a -> forall b. b -> c1
       In the pattern: _ :: forall a. a -> forall b. b -> c
       In an equation for ‘f’:
           f (_ :: forall a. a -> forall b. b -> c) = ()
     • Relevant bindings include
         f :: (forall a. a -> forall b. b -> c1) -> () (bound at
 Bug.hs:7:1)
   |
 7 | f (_ ::  forall a. a -> forall b. b -> c)  = ()
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16033#comment:1>
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