[GHC] #13509: Perplexing type error

[GHC]

#13509: Perplexing type error
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism
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 simonpj):

 Even shorter repro case
 {{{
 {-# LANGUAGE UnboxedTuples, MagicHash #-}
 module T13509 where

 import GHC.Exts
 import Data.Word

 selectPower :: Word# -> (# Word#, Word# #)
 selectPower base = go base
   where
     go :: Word# -> (# Word#, Word# #)
     go pw = let (# n, pw2n #) = go pw
             in (# n `timesWord#` 2##, pw2n #)
 }}}
 This compiles.  Commenting out the signature for `go` makes it fail.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13509#comment:4>
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