[GHC] #10789: Notify user when a kind mismatch holds up a type family reduction

[GHC]

#10789: Notify user when a kind mismatch holds up a type family reduction
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        Reporter:  goldfire          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:  newcomer
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Changes (by goldfire):

 * keywords:   => newcomer
 * milestone:   => 7.12.1


Comment:

 A little coding in `TcErrors` should do it. Happy to advise someone who
 wants to take this on.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789#comment:2>
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