[GHC] #12557: Regression in type inference with RankNTypes

[GHC]

#12557: Regression in type inference with RankNTypes
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        Reporter:  slindley          |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:  invalid           |  ImpredicativeTypes
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by slindley):

 It does look like there's a bug in the error message. The message I want
 to see here is that t must be monomorphic, not that it is rigid.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12557#comment:4>
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