[GHC] #12485: -package-db flags now need to be sorted by dependency order

[GHC]

#12485: -package-db flags now need to be sorted by dependency order
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        Reporter:  niteria           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.3
       Component:  Package system    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  phab:D2450,
       Wiki Page:                    |  phab:D2514
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Comment (by simonmar):

 > And this is what I get for not being involved in the initial discussion
 ;) But it is hard for me to see how you could actually do this, because
 you can always end up in a situation where you have two separate packages
 with the same symbol name, and now you have to pick one.

 That's true.  In that case, either
 * The ABIs are the same, and we can just pick one, or
 * The ABIs are different, in which case we can either
   * report an error, or
   * Pick one to keep, and throw away everything that depends on the
 dropped one

 Reporting an error is bad - when we had something similar before (7.2? I
 forget exactly) you could get into a state where GHC fails to start
 because you have some conflict in your package database.  That's why the
 GHC tries really hard to find a consistent set of packages by throwing
 things away.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12485#comment:21>
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