[GHC] #15135: Overlapping typeclass instance selection depends on the optimisation level

[GHC]

#15135: Overlapping typeclass instance selection depends on the optimisation level
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        Reporter:  nicuveo           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.2
      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 baramoglo):

 Here is an example that exhibits the same bug (I think) in a single file:


 {{{
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiParamTypeClasses #-}

 data A = A
   deriving (Show)

 data B a = B a
   deriving (Show)

 class Project a b where
   project :: b -> Maybe a

 instance {-# OVERLAPPING #-} Project a b where
   project _ = Nothing

 instance {-# OVERLAPPING #-} Project a a where
   project = Just

 instance {-# OVERLAPPING #-} Project a b => Project a (B b) where
   project (B a) = project a

 main = print (project (B A) :: Maybe A)
 }}}

 Prints `Just A` when compiled with `-O0` and `Nothing` when compiled with
 `-O1`.

 Note that the first instance should really say `{-# OVERLAPPABLE #-}`
 (AFAIU). If I change to that, the bug goes away.

 Let me know if you think this should be filed as a different bug.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15135#comment:3>
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