[GHC] #14440: Duplicate type family instances are permitted

[GHC]

#14440: Duplicate type family instances are permitted
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AntC):

 Replying to [ticket:14440 RyanGlScott]:
 > My intuition screams "no", since if we offer //class// instance
 coherence, ...

 And the short answer to that is we '''don't''' offer and can't guarantee
 class instance coherence/consistency, if you're talking about FunDeps. See
 SPJ on [https://ghc.haskell.org/trac/ghc/ticket/10675#comment:15 'bogus
 consistency check']. (Sorry, but there's plenty of code abusing that
 consistency check, pioneered in GHC in the 2004 HList paper. Contrast Hugs
 never allowed such abuse. There is [https://github.com/ghc-proposals/ghc-
 proposals/pull/56 a way out]. Please upvote.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14440#comment:2>
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