[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 simonpj):

 See Section 3.6 of the [https://www.microsoft.com/en-
 us/research/publication/closed-type-families-with-overlapping-equations/
 Closed type families] paper.  Having identical RHSs is a degenerate case
 of Definition 10.

 But perhaps it's so degenerate that it should be reported with a warning;
 I'm not sure.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14440#comment:3>
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