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

[GHC]

#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas
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              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:              |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"1ae5fa451f4f554e0d652d55f9312a585188ce13/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1ae5fa451f4f554e0d652d55f9312a585188ce13"
 Complete work on new OVERLAPPABLE/OVERLAPPING pragmas (Trac #9242)

 * Deprecate -XOverlappingInstances

 * Update test suite.  Several tests even had entirely unnecessary
   uses of -XOverlappingInstances

 * Update user manual with a careful description of the instance
   resolution story

 * Fix an outright bug in the handling of duplidate instances in GHCi,
   which are meant to silently overwrite the earlier duplicate. The
   logic was right for family instances but was both more complicated,
   and plain wrong, for class instances.  (If you are interested, the
   bug was that we were eliminating the duplicate from the InstEnv, but
   not from the [ClsInst] held in tcg_insts.)  Test is ghci044a.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:28>
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