[GHC] #15125: Typeclass instance selection depends on the optimisation level

[GHC]

#15125: Typeclass instance selection depends on the optimisation level
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        Reporter:  nicuveo           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      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:                    |
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Comment (by nicuveo):

 I am afraid I still see this behaviour in 8.4.2...

 I replaced INCOHERENT with OVERLAPPABLE, built my small example with
 --resolver nightly-2018-05-09, and in both cases it shortcuts and picks
 the general instance and prints 0.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15125#comment:6>
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