[GHC] #9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas

[GHC]

#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |            Owner:  diatchki
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"dff0623d5ab13222c06b3ff6b32793e05b417970/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="dff0623d5ab13222c06b3ff6b32793e05b417970"
 Implement the final change to INCOHERENT from Trac #9242

 The change here is to make INCOHERENT slightly more permissive:

   if the selected candidate is incoherent
   then ignore all unifying candidates

 This allows us to move the {-# INCOHERENT #-} pragma from
   from   instance Typeable (f a)
   to     Typeable (n:Nat) and Typable (s:Symbol)
 where it belongs, and where Trac #9242 said it should be.

 I don't think this will affect anyone.

 I've updated the user manual.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:35>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler