[GHC] #10602: ghc panic: Template variable unbound in rewrite rule when compiling with -O2

[GHC]

#10602: ghc panic: Template variable unbound in rewrite rule when compiling with
-O2
-------------------------------------+-------------------------------------
        Reporter:  pacak             |                   Owner:  bgamari
            Type:  bug               |                  Status:  infoneeded
        Priority:  highest           |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.10.2-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by thomie):

 I attached an alternative test that also fails with a devel2 build. `-fno-
 spec-constr` makes the problem go away, as do a few other things (see
 `T10602.hs`).

 Maybe this debug message is of help to someone:
 {{{
 $ ./ghc-devel2/inplace/bin/ghc-stage2 T10602.hs
 [2 of 2] Compiling T10602           ( T10602.hs, T10602.o )
 SpecConstr
   Function ‘$wgo_s13x’ has one call pattern, but the limit is 0
   Use -fspec-constr-count=n to set the bound
   Use -dppr-debug to see specialisations
 ghc-stage2: panic! (the 'impossible' happened)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10602#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler