[GHC] #12881: GHC 8.0.2 regression involving OVERLAP annotations

[GHC]

#12881: GHC 8.0.2 regression involving OVERLAP annotations
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.2-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2760
       Wiki Page:                    |
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Comment (by rwbarton):

 It is possible to write useful potentially-overlapping instances without
 marking them as overlapping, e.g.
 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}

 class C a where
   def :: a
   def = undefined

 instance C (Int, b)
 instance C (a, Char)
 }}}
 But happily GHC HEAD already accepts this case, somehow. (Maybe someone
 with 8.0.2-rc1 can confirm.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12881#comment:10>
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