[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
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 (See the attached files for a minimal case.)

 A file A defines a typeclass, and gives an incoherent instance for all
 types a, and exports a function relying on said typeclass. A file B
 defines some data types, makes them specific instances of that class.
 Which instance ends up being picked depends on the optimisation level
 they're compiled with.

 //A.hs//
 {{{#!hs
 class A a where
   someValue :: a -> Maybe Int

 instance {-# INCOHERENT #-} A a where
   someValue = const Nothing

 getInt :: A a => a -> Int
 getInt x = fromMaybe 0 $ someValue x
 }}}

 //B.hs//
 {{{#!hs
 data B = B Int

 instance A B where
   someValue (B x) = Just x

 getBInt :: Int
 getBInt = getInt $ B 42
 }}}


 //Main.hs//
 {{{#!hs
 main = print getBInt
 }}}

 **Upon compiling with -O0, this prints 42; upon compiling with -O2, this
 print 0.**

 (Interestingly, if the redundant class constraint is removed from getInt,
 then it always prints 0.)

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