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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:

{-# 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)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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