[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:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Ah. Now I understand better. Instantiating the shape is just incidental,
 done so that unification succeeds at all. The real action is in solving
 for any metavars in the data constructor type, and this unification does
 ''not'' affect anything about the head shape.

 Yes, that sounds reasonable.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14111#comment:10>
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