[GHC] #10689: compiling singletons-1.1.2.1 as -O1 -fspec-constr fails as 'Template variable unbound in rewrite rule'

[GHC]

#10689: compiling singletons-1.1.2.1 as -O1 -fspec-constr fails as 'Template
variable unbound in rewrite rule'
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        Reporter:  slyfox            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Changes (by slyfox):

 * Attachment "List.hs" added.

 a tiny part of singletons-1.1.2.1/src/Data/Singletons/Prelude/List.hs
 required to trigger a bug

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10689>
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