[GHC] #11473: Levity polymorphism checks are inadequate

[GHC]

#11473: Levity polymorphism checks are inadequate
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        Reporter:  simonpj           |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism, TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Richard Eisenberg <eir@…>):

 In [changeset:"aade111248dce0834ed83dc4f18c234967b32024/ghc"
 aade1112/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="aade111248dce0834ed83dc4f18c234967b32024"
 Fix #11473.

 I've added a check in the zonker for representation polymorphism.
 I don't like having this be in the zonker, but I don't know where
 else to put it. It can't go in TcValidity, because a clever enough
 user could convince the solver to do bogus representation polymorphism
 even though there's nothing obviously wrong in what they wrote.
 Note that TcValidity doesn't run over *expressions*, which is where
 this problem arises.

 In any case, the check is simple and it works.

 test case: dependent/should_fail/T11473
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11473#comment:11>
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