[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:  8.0.2
       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:                    |
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Comment (by ezyang):

 A few things:

 First, niteria, I'm curious why you need top be able to pass the package
 databases out of order. If I understand correctly, neither Stack nor Cabal
 use the databases this way. Indeed, when I wrote this patch, I assumed the
 databases would be in the right order because that's how shadowing works:
 later databases shadow earlier ones.

 Second, it's impossible to do ABI sanity checking in the way it is done
 today if I can't assume things are in the right order. When the db order
 was assumed to be specified correctly, I could assume all references to
 p-0.1-xxx refer to the topmost package with this IPID in the Stack; if the
 ABIs mismatch then I should remove everyone below me. But if there might
 be a reference to that particular package in a later package database,
 this assumption doesn't hold.

 Assuming that we DO want databases to be specified in any order (do we?!)
 here's what I think we should do: We NEVER remove packages because
 something got shadowed. Either the shadower is ABI compatible (in which
 case we never need to eliminate anything), OR we immediately error
 (because we have no idea which ones to eliminate.) I don't know if this
 brings back enough shadowing to fix bootstrapping though; will have to
 test.

 #12518 seems related; it also requests that we error on shadowing.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12485#comment:7>
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