compiling singletons-1.1.2.1 as -O1 -fspec-constr fails as 'Template variable unbound in rewrite rule'

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.

Trac metadata

Trac field	Value
Version	7.10.2-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire, simonpj
Operating system
Architecture