[GHC] #12911: Levity polymorphism check eliminates non-levity-polymorphic data constructor

1 Dec 2016

      #12911: Levity polymorphism check eliminates non-levity-polymorphic data
constructor
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  LevityPolymorphism                 |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When I say

 {{{
 data X where
   MkX :: forall (a :: TYPE r). (a -> a) -> X
 }}}

 I get

 {{{
     • A representation-polymorphic type is not allowed here:
         Type: a
         Kind: TYPE r
     • In the definition of data constructor ‘MkX’
       In the data type declaration for ‘X’
 }}}

 But that's silly, because the type is not levity-polymorphic!

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12911
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