[GHC] #8566: Given kind equalities are discarded

[GHC]

#8566: Given kind equalities are discarded
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        Reporter:  dreixel           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  polykinds/T8566   |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 I'm fine with your last suggestion, but I wonder if there's a different
 way: instead of rejecting the constructor, what if the "bad error
 messages" suggest the nature of the problem? For example, if a type error
 occurs after dropping an unusable given kind equality, that error message
 could say what kind equality was dropped and why. (This is somewhat like
 error messages that warn about ambiguous variables.) I would imagine that
 the folks who write the weirdly-existential constructors would be able to
 understand such a message and respond appropriately. Would this be an
 engineering challenge (that is, remembering the dropped given equality,
 just for error reporting)?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8566#comment:13>
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