[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 AntC):

 > This seems inconsistent with the ​documented behaviour of incoherent
 instances?

 The naming is deliberate: only use `INCOHERENT` to diagnose behaviour you
 don't understand; never rely on ghc consistently picking an instance.

 The behaviour you're seeing in 8.4.2 is following the documentation. The
 critical part of that doco is

 >>  Eliminate any candidate **IX** for which ''both'' of the following
 hold:

 The second sub-bullet doesn't hold, because neither instance is marked
 ''overlapping''/''overlappable''.

 So I'd expect to always pick the more general instance, marked
 `INCOHERENT`.

 Then I agree the optimisation level shouldn't influence instance
 selection. I guess that in optimising for module B, ghc forces the
 instance selection. Never the less, just don't use `INCOHERENT`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15125#comment:3>
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