[GHC] #9820: Apparently inconsistent behaviour in the presence of OverlappingInstances

[GHC]

#9820: Apparently inconsistent behaviour in the presence of OverlappingInstances
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       Reporter:  kosmikus           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 This report is inspired by http://stackoverflow.com/questions/27051640
 /strange-behavior-when-adding-constraint-to-instance

 I'm not sure if it is a bug, but it certainly feels strange and
 inconsistent to me, so I'd like to get some clarification.

 Here's a self-contained example of the behaviour:
 {{{#!hs
 {-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
 module Overlap where

 class C a where
   foo :: a -> a

 instance C a where
   foo = id

 instance C Int where
   foo = (+1)

 -- Note that this one does not mention class C anywhere:
 class D a where
   bar :: a -> a

 instance D a where
   bar = foo -- this works and picks the 'C a' instance always

 -- The following does not compile without IncoherentInstances:
 {-
 baz :: a -> a
 baz = foo
 -}
 }}}

 I'm surprised that the definitions of `bar` and `baz` are treated
 differently. Shouldn't they either both require `IncoherentInstances`, or
 both work?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9820>
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