[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'
-------------------------------------+-------------------------------------
              Reporter:  slyfox      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2-rc2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 First observed on singletons-1.1.2.1 package ghc-7.10.2.

 I've tried to distill it down to a single file.

 The file manages to crash both GHC and GHCi.

 GHCi:
 {{{
 $ inplace/bin/ghc-stage2 -hide-all-packages -package=base --interactive
 List.hs
 GHCi, version 7.11.20150723: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling List             ( List.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.11.20150723 for x86_64-unknown-linux):
         floatExpr tick break<2>()

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 GHC:
 {{{
 $ inplace/bin/ghc-stage2 -hide-all-packages -package=base --make List.hs
 -dsuppress-all -dsuppress-uniques
 [1 of 1] Compiling List             ( List.hs, List.o )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.11.20150723 for x86_64-unknown-linux):
         Template variable unbound in rewrite rule
   m
   [a_afdP, x, m, ipv, ipv, sc, sc, sc, sg]
   [a_afdP, x, m, ipv, ipv, sc, sc, sc, sg]
   [TYPE a_afdP, TYPE x, TYPE Let1627448493XsSym4 x m ipv ipv, sc,
    (SCons @~ ... sc sc) `cast` ...]
   [TYPE a_afdP, TYPE x, TYPE Let1627448493XsSym4 x ipv ipv ipv, sc,
    (SCons @~ ... ipv ipv) `cast` ...]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 The "minimal" single-file sample is attached.

 If you need original selfcontained reproducer (79 modules, template
 haskell code) this is the version i've started with:
 http://code.haskell.org/~slyfox/unbound-template-var.tar.gz
 Adjusting path to GHC in '''./trigger-a-bug.bash''' should be enough.

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