[GHC] #13929: GHC panic with levity polymorphism

[GHC]

#13929: GHC panic with levity polymorphism
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        Reporter:  vagarenko         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.2
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:  TypeInType,
                                     |  LevityPolymorphism
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 vagarenko):

 Replying to [comment:10 Ben Gamari <ben@…>]:

 Can you please elaborate why you think GHC should reject this.
 This error message doesn't say much
 {{{
 T13929.hs:29:27: error:
     A levity-polymorphic type is not allowed here:
       Type: GUnboxed f rf
       Kind: TYPE rf
     In the type of expression: gunbox x

 T13929.hs:29:37: error:
     A levity-polymorphic type is not allowed here:
       Type: GUnboxed g rg
       Kind: TYPE rg
     In the type of expression: gunbox y
 }}}

 As I mentioned in comment 8:
 >>To be clear, I don't think (although could be wrong) this program should
 compile; afterall, the type of gunbox requires that we represent a levity
 polymorphic result, which we cannot do. Hence, the real bug here is the
 panic instead of a proper type error.
 >I think levity polymorphism paper says you can't have functions with
 levity-polymorphic parameters, levity-polymorphic results are OK.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13929#comment:11>
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