[GHC] #13248: Allow an injective type family RHS to be another injective type family

[GHC]

#13248: Allow an injective type family RHS to be another injective type family
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        Reporter:  mniip             |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      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 simonpj):

 If I recall correctly this is by design.  It's explained in
 [https://www.microsoft.com/en-us/research/publication/injective-type-
 families-haskell/ Section 4.1 of the paper].

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13248#comment:1>
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