[GHC] #11386: Improve error message

[GHC]

#11386: Improve error message
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        Reporter:  Iceland_jack      |                Owner:
            Type:  task              |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10362            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 If you had written
 {{{
 class T A b
 }}}
 Then the error would have been fine.  What is special here?  Just that
 it's using a type constructor that has built-in syntax.

 I expect I could add a special case to dis-allow such class declarations,
 and I agree this one is strange because of the `%` signs.  But is it
 really worth the trouble?  Will people do this much?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11386#comment:4>
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