[GHC] #12945: Backpack signature matching doesn't pick up orphan instances

[GHC]

#12945: Backpack signature matching doesn't pick up orphan instances
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        Reporter:  ezyang            |                Owner:  ezyang
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
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 Edward Z. Yang <ezyang@…>):

 In [changeset:"62332f36b62431ddb9ab3c97365288c7d3fc2d39/ghc" 62332f3/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="62332f36b62431ddb9ab3c97365288c7d3fc2d39"
 Setup tcg_imports earlier during signature matching, so orphans are
 visible.

 Summary:
 Previously, we updated tcg_imports after doing all of the
 actual matching, which was fine for outputting the interface,
 but not good enough for checking if all type classes were
 implemented; we weren't treating orphans as visible (when
 they needed to be.)

 Fixes #12945.

 Signed-off-by: Edward Z. Yang <ezyang at cs.stanford.edu>

 Test Plan: validate

 Reviewers: simonpj, austin, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2802

 GHC Trac Issues: #12945
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12945#comment:1>
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