[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
-------------------------------------+-------------------------------------
        Reporter:  niteria           |                Owner:
            Type:  bug               |               Status:  patch
        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:D2613
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonmar):

 @duncan in practice the keys will often be strong enough.  However, there
 are a couple of reasons why we can end up with package builds that have
 the same key but are incompatible.

 * Prior to GHC 8.0.2 non-determinism meant that the key did not determine
 the ABI

 * We don't want the key to be a function of the source code, because that
 would entail too much recompilation during development.  For this reason,
 the GHC build system picks deterministic package keys, and I believe Cabal
 doesn't hash the source code to compute the package key (but I just gave
 up trying to confirm that after trawling through the Cabal source code for
 a few minutes which seems to have got terrifyingly complex since I last
 looked at it!)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12485#comment:35>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler