[GHC] #13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and ExistentialQuantification

[GHC]

#13242: Panic "StgCmmEnv: variable not found" with ApplicativeDo and
ExistentialQuantification
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        Reporter:  ljli              |                Owner:  simonmar
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.3
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonmar):

 @simonpj, so the new code doesn't bring constraints into scope from
 existential bindings, even in statements that sequentially follow the
 existential pattern match.  e.g.

 {{{
 Prelude> data T where A :: forall a . Eq a => a -> T
 Prelude> :set -XApplicativeDo
 Prelude> do { A x <- undefined; y <- return (); return (x == x) }

 <interactive>:7:48: error:
     • Could not deduce (Eq a) arising from a use of ‘==’
       from the context: Monad f
         bound by the inferred type of it :: Monad f => f Bool
         at <interactive>:7:1-56
       Possible fix:
         add (Eq a) to the context of
           the inferred type of it :: Monad f => f Bool
     • In a stmt of a 'do' block: return (x == x)
       In the expression:
         do A x <- undefined
            y <- return ()
            return (x == x)
       In an equation for ‘it’:
           it
             = do A x <- undefined
                  y <- return ()
                  return (x == x)
 }}}

 I understand why - because the implementation specifically only binds Ids
 - but it's somewhat surprising.  Is this fixable, or should I try to
 document it?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13242#comment:16>
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