[GHC] #10182: lookupIfaceGlobal crash with SOURCE import

[GHC]

#10182: lookupIfaceGlobal crash with SOURCE import
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        Reporter:  simonpj           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                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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Comment (by ezyang):

 I can reproduce with an even more minimized test-case, on a stage 2 quick
 build:

 {{{
 module T8743 where
 class ToRow a
 instance ToRow ()
 }}}

 {{{
 module T8743 where
 import {-# SOURCE #-} T8743 ()
 class ToRow a
 instance ToRow () where
 }}}

 With some more tracing you can get a clue: `tcIfaceGlobal` is being called
 because GHC is trying to report a duplicate instance error. That certainly
 should not happen! So in one-shot compilation, GHC is seeing an instance
 in its environment that is NOT supposed to be there.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10182#comment:4>
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