[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):

 Yes, I'm using the version in the tarball; I simplified it for the purpose
 of the bug description. And my apologies: I double-checked, and indeed,
 the code in the tarball works for 8.4.2; my stack setup was wrong. Sorry
 about that!

 ...however, sadly, there is the other bug I highlight in comment:6.

 If you replace //A.hs// to read:
 {{{#!hs
 instance A a where
   someValue = const Nothing
 }}}

 and //B.hs// to read:
 {{{#!hs
 instance {-# OVERLAPPING #-} A B where
   someValue (B x) = Just x
 }}}

 Then the same behaviour is back with 8.4.2: with -O0, it prints 42, with
 -O2, it prints 0.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15125#comment:9>
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