[GHC] #15740: Type family with higher-rank result is too accepting

[GHC]

#15740: Type family with higher-rank result is too accepting
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  TypeFamilies,
                                     |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15793            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Simon appears to have stumbled upon this separately, in his ongoing work
 captured in Phab:D5305. The fix appears to be a small change to
 `kcTyFamEqnRhs` to instantiate only invisible arguments that are within
 the arity of the tyfam; right now, it wrongly instantiates ''all''
 invisible arguments.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15740#comment:6>
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