[GHC] #12630: Assertion failed with BuildFlavour = devel2

[GHC]

#12630: Assertion failed with  BuildFlavour = devel2
-------------------------------------+-------------------------------------
        Reporter:  pacak             |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 OK this is `substTy` called from `SimplEnv` i.e. in the simplifier.

 Again it's unlikely to be causing a problem but I'd like to nail what's
 going on.  Do you have time to make a reproducible case?  It doesn't have
 to be terribly small, but having few dependencies is helpful.  You 8-file
 blob might do.

 Thanks

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12630#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler