[GHC] #4012: Compilation results are not deterministic

[GHC]

#4012: Compilation results are not deterministic
-------------------------------------+-------------------------------------
        Reporter:  kili              |                Owner:  niteria
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  6.12.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:  11362             |             Blocking:
 Related Tickets:  #10424            |  Differential Rev(s):  Phab:D910,
                                     |  Phab:D1073, Phab:D1133, Phab:D1192,
                                     |  Phab:D1268, Phab:D1360, Phab:D1373,
       Wiki Page:                    |  Phab:D1396, Phab:D1457, Phab:D1468,
  DeterministicBuilds                |  Phab:D1487, Phab:D1504, Phab:D1508
-------------------------------------+-------------------------------------

Comment (by Bartosz Nitka <niteria@…>):

 In [changeset:"82538f65f48f370764691264c3c71b975fd43e16/ghc"
 82538f65/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="82538f65f48f370764691264c3c71b975fd43e16"
 Kill varSetElems in injImproveEqns

 We want to remove varSetElems at the source level because it
 might be a source of nondeterminism. I don't think it introduces
 nondeterminism here, but it's easy to do the same thing
 deterministically for the same price.

 instFlexiTcS :: [TKVar] -> TcS (TCvSubst, [TcType])
 instFlexiTcS currently gives the range of the produced substitution
 as the second element of the tuple, but it's not used anywhere
 right now. If it started to be used in the code I'm modifying
 it would cause nondeterminism problems.

 Test Plan: ./validate

 Reviewers: austin, goldfire, bgamari, simonmar, simonpj

 Reviewed By: simonpj

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2149

 GHC Trac Issues: #4012
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4012#comment:154>
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