[GHC] #10112: Desugaring of do-syntax intros unification var with -XRebindableSyntax

[GHC]

#10112: Desugaring of do-syntax intros unification var with -XRebindableSyntax
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              Reporter:  heisenbug   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 When compiling below snippet with GHC 7.8.x (and I believe v7.10.x)

 {{{
 {-# LANGUAGE RankNTypes, RebindableSyntax #-}

 import qualified Prelude as P

 (>>=) :: a -> ((forall b . b) -> c) -> c
 a >>= f = f P.undefined
 return a = a
 fail s = P.undefined

 t1 = 'd' >>= (\_ -> 'k')

 t2 = do _ <- 'd'
         'k'

 main = P.putStrLn [t1, t2]
 }}}

 we get this error:
 {{{
 Ztest.hs:12:9:
     Cannot instantiate unification variable `t0`
     with a type involving foralls: forall b. b
       Perhaps you want ImpredicativeTypes
     In a stmt of a 'do' block: _ <- 'd'
     In the expression:
       do { _ <- 'd';
            'k' }
     In an equation for `t2`:
         t2
           = do { _ <- 'd';
                  'k' }
 Failed, modules loaded: none.
 }}}

 In GHC HEAD (and v7.6.x) the error does not appear. Nevertheless I'll file
 this bug for addition of a regression test.

 Discussion here: https://mail.haskell.org/pipermail/ghc-
 devs/2015-February/008383.html

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10112>
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