[GHC] #9288: Type class overlapping instances check doesn't understand type equality

[GHC]

#9288: Type class overlapping instances check doesn't understand type equality
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        Reporter:  ezyang                       |            Owner:
            Type:  bug                          |           Status:  new
        Priority:  low                          |        Milestone:
       Component:  Compiler (Type checker)      |          Version:  7.9
      Resolution:                               |         Keywords:
Operating System:  Unknown/Multiple             |     Architecture:
 Type of failure:  GHC accepts invalid program  |  Unknown/Multiple
       Test Case:                               |       Difficulty:
        Blocking:                               |  Unknown
                                                |       Blocked By:
                                                |  Related Tickets:
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Comment (by aavogt):

 A simpler example where ghc-7.8.2 doesn't detect overlapping instances
 until they are used

 {{{
 {-# LANGUAGE FlexibleInstances #-}
 class C a where c :: a
 instance C a where c = undefined
 instance C Int where c = 2
 -- ghc doesn't complain about overlapping instances
 -- unless you uncomment c1
 -- c1 = c :: Int
 c2 = c :: Double
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9288#comment:1>
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