[GHC] #11207: GHC cannot infer injectivity on type family operating on GHC.TypeLits' Nat, but can for equivalent type family operating on user-defined Nat kind

[GHC]

#11207: GHC cannot infer injectivity on type family operating on GHC.TypeLits' Nat,
but can for equivalent type family operating on user-defined Nat kind
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        Reporter:  duairc            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
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 duairc):

 You're right, they are subtley different. But I can't "pattern-match" on
 (n + 1) on the LHS with GHC's TypeLits, because + is a type family and not
 a constructor. If there was some other way I could accomplish this then I
 would be satisfied with that for now. But obviously if we could get
 generalised injectivity that we be awesome.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11207#comment:3>
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