[GHC] #9432: IncoherentInstances are too restricted

[GHC]

#9432: IncoherentInstances are too restricted
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              Reporter:  danilo2     |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  high        |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by danilo2):

 Maybe we should treat it as a bug? I've got here a small example showing
 that it really breaks how `OverlappingInstances` are working:

 {{{#!haskell
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE OverlappingInstances #-}
 {-# LANGUAGE IncoherentInstances #-} -- the flag is niot needed by the
 example

 module Main where
 import Data.Typeable


 class CTest a b | a -> b where
     cTest :: a -> b
     cTest = undefined


 -- this example is choosen even if more specific is available!
 instance out~a => CTest a out where
     cTest = id

 instance CTest Int String where
     cTest _ = "test"


 main = do
     print $ typeOf $ cTest (5::Int)
 }}}

 The instance `CTest a out` even if more specific (`CTest Int String`) is
 in scope, which just breaks how `OverlappingInstances` work. If we disable
 the `IncoherentInstances` flag, the right one is selected.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9432#comment:1>
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