[GHC] #5821: SPECIALISE fails with a cryptic warning

[GHC]

#5821: SPECIALISE fails with a cryptic warning
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              Reporter:  rl          |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.5
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Incorrect   |       Blocked By:
  warning at compile-time            |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by thomie):

 In HEAD, the error message for the program from the description has
 changed to:
 {{{
 $ ghc-7.9.20141125 T5821.hs
 [1 of 1] Compiling T5821            ( T5821.hs, T5821.o )

 T5821.hs:10:1: Warning:
     RULE left-hand side too complicated to desugar
       Optimised lhs: case cobox_ang
                      of _ [Occ=Dead] { GHC.Types.Eq# cobox ->
                      (foo @ Int $dNum_anf)
                      `cast` (<Int>_R -> Sub cobox :: (Int -> T Int) ~R#
 (Int -> Bool))
                      }
       Orig lhs: case cobox_ang of cobox_ang { GHC.Types.Eq# cobox ->
                 (foo @ Int $dNum_anf)
                 `cast` (<Int>_R -> Sub cobox :: (Int -> T Int) ~R# (Int ->
 Bool))
                 }
 }}}


 Due to commit b34068144ec3d7bfe4279b16ad16d54dd46f1c5a:
 {{{
 Author: Simon Peyton Jones <>
 Date:   Thu Mar 13 08:36:28 2014 +0000

     A bit more tracing to do with SPECIALISE pragmas
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5821#comment:7>
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