[GHC] #12064: tcIfaceGlobal error with existentially quantified types

15 May 2016

      #12064: tcIfaceGlobal error with existentially quantified types
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           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  hs-boot        |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following program:

 {{{
 -- A.hs-boot
 module A where
 data D

 -- B.hs
 module B where
 import {-# SOURCE #-} A
 class K a where
   method :: D -> a

 -- A.hs
 {-# LANGUAGE ExistentialQuantification #-}
 module A where
 import A
 data D = forall n. K n => D
 }}}

 We get a `tcIfaceGlobal` error in this case (one-shot of course.) I
 reduced this case out of prog006.

 So for some reason we're tugging on `K` too early—that's as far as I know
 at this point.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12064
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