[GHC] #15594: --abi-hash with Backpack incorrectly loads modules from dependent packages

[GHC]

#15594: --abi-hash with Backpack incorrectly loads modules from dependent packages
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      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):  Phab:D5123
       Wiki Page:                    |
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Description changed by ezyang:

Old description:

> Repro at: https://github.com/alexbiehl/cabal-backpack-register-repro

New description:

 Repro at: https://github.com/alexbiehl/cabal-backpack-register-repro

 If you use Backpack with data families, you might fail during `--abi-hash`
 with:

 {{{
 cabal:
 '/nix/store/m338klajhqlw7v4jd61fiqd82wx305fj-ghc-8.4.3-with-
 packages/bin/ghc'
 exited with an error:
 Failed to load interface for ‘Stuff’
 no unit id matching ‘backpack-trans-0.1.0.0-L6CFTQZAAWWFpCQD2NXR4W-indef’
 was
 found
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15594#comment:3>
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