[GHC] #7567: invalidateModSummaryCache throws exception if ms_hs_date is 0

[GHC]

#7567: invalidateModSummaryCache throws exception if ms_hs_date is 0
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        Reporter:  edsko             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  high              |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.6.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by bgamari):

 Is it not safe to assume that the users environment will be non-broken
 enough to have functional mtimes? Under what circumstances did this
 happen?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7567#comment:4>
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