[GHC] #7395: DefaultSignatures conflict with default implementations

[GHC]

#7395: DefaultSignatures conflict with default implementations
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        Reporter:  cgaebel           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.1
      Resolution:                    |             Keywords:
                                     |  DefaultSignatures
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  7346              |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by aspiwack):

 I found myself, today, in a situation where I wanted to have two default
 signatures, something like:

 {{{#!haskell
 {-# LANGUAGE DefaultSignatures #-}

 module Test where

 class Foo a where
   foo :: a

   default foo :: (a~Int) => a
   foo = 0

   default foo :: (a~Bool) => a
   foo = False
 }}}

 The practical situation was that the second default implementation
 requires an extra `MonadIO m` constraint and is more efficient.

 My proposal is to resolve default implementation like overlapping
 instances: the default with the most specific type amongst the well-typed
 ones is used.

 The compiler can check, when creating the class that default
 implementations of a same method either don't unify or are strictly
 ordered with the specialisation order.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7395#comment:19>
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