[GHC] #12792: Wrong error message when using a data type as a class instance head

[GHC]

#12792: Wrong error message when using a data type as a class instance head
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        Reporter:  nomeata           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 Do we need full kind-checking?

 The renamer already knows the concrete TyCon that is heading the instance.
 Shouldn’t `isClassTyCon` be sufficient here?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12792#comment:2>
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