[GHC] #13594: Typechecker behavior w.r.t. BangPatterns and nested foralls has changed in 8.2

[GHC]

#13594: Typechecker behavior w.r.t. BangPatterns and nested foralls has changed in
8.2
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
  (Type checker)                     |
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 Try running this code in GHCi:

 {{{
 λ> :set -XBangPatterns -XRankNTypes  -XTypeFamilies
 λ> let x :: forall a . a ~ Integer => forall b. b ~ Integer => (a, b); !x
 = (1, 2)
 }}}

 In GHC 8.0.1 and 8.0.2, this works. But in GHC 8.2.1:

 {{{
 <interactive>:3:74:
     Couldn't match expected type ‘forall a.
                                   (a ~ Integer) =>
                                   forall b. (b ~ Integer) => (a, b)’
                 with actual type ‘(Integer, Integer)’
     In the expression: (1, 2)
     In a pattern binding: !x = (1, 2)
 }}}

 If you put this code into a source file:

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

 x :: forall a . a ~ Integer => forall b. b ~ Integer => (a, b)
 !x = (1, 2)
 }}}

 Then it also works in GHC 8.0.1. and 8.0.2, but it errors on GHC 8.2 (this
 time with a different error):

 {{{
 GHCi, version 8.2.0.20170413: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:6:1: error:
     Overloaded signature conflicts with monomorphism restriction
       x :: forall a. a ~ Integer => forall b. b ~ Integer => (a, b)
   |
 6 | x :: forall a . a ~ Integer => forall b. b ~ Integer => (a, b)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13594>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler