[GHC] #5610: Improve "Unacceptable argument type in foreign declaration" error message

[GHC]

#5610: Improve "Unacceptable argument type in foreign declaration" error message
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        Reporter:  bgamari                       |            Owner:
            Type:  feature request               |           Status:  new
        Priority:  high                          |        Milestone:  7.4.1
       Component:  Compiler (Type checker)       |          Version:
      Resolution:                                |  7.6.1-rc1
Operating System:  Unknown/Multiple              |         Keywords:
 Type of failure:  Incorrect warning at          |     Architecture:
  compile-time                                   |  Unknown/Multiple
       Test Case:                                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Comment (by archblob):

 I have time to work on this and do a little refactoring to improve the
 error messages. The comment history is a little confusing and I'm not
 really sure that a better error message is the only thing requested here,
 and if the whole importing of constructors for foreign declarations story
 is also in need of fixing. It's the same story with #7243, if someone
 could make things clear for me I would be happy to start working.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5610#comment:20>
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