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

[GHC]

#12064: tcIfaceGlobal error with existentially quantified types
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        Reporter:  ezyang            |                Owner:  ezyang
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  hs-boot
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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Description changed by ezyang:

@@ -17,1 +17,1 @@
- import A
+ import B

New description:

 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 B
 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#comment:2>
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