[GHC] #14111: strange error when using data families with levity polymorphism and unboxed sums and data families

[GHC]

#14111: strange error when using data families with levity polymorphism and unboxed
sums and data families
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        Reporter:  carter            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:  13737             |             Blocking:
 Related Tickets:  #14457            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 There is precedent with the strategy in comment:21. Note the arity check
 toward the top of `tcFamTyPats`. This check is fully redundant, but it
 gives a nice arity error (simple to understand!) instead of an obscure
 kind error.

 So, I would say to go with comment:21, with a comment (not sure if it
 needs a Note) explaining why do the eager check outside of
 `checkValidDataCon`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14111#comment:22>
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