[GHC] #11106: Turning on optimizer changes behavior in 7.10.3

[GHC]

#11106: Turning on optimizer changes behavior in 7.10.3
-------------------------------------+-------------------------------------
        Reporter:  dsf               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by nomeata):

 Further simplification, removal of derived instances and spreading things
 out over multiple modules shows that `-O0` has to apply to these four
 lines only for the bug to occur:
 {{{
 instance Eq ReportElemID where (==) a b = (==) (unReportElemID a)
 (unReportElemID b)
 instance Ord ReportElemID where compare a b = compare (unReportElemID a)
 (unReportElemID b)
 instance Enum ReportElemID where
       toEnum = (ReportElemID . toEnum)
       fromEnum = (fromEnum . unReportElemID)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11106#comment:12>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler