[GHC] #13410: GHC HEAD regression: Template variable unbound in rewrite rule

[GHC]

#13410: GHC HEAD regression: Template variable unbound in rewrite rule
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * version:  8.1 => 8.0.1


Comment:

 Oh dear, there's actually a distinction between the attached program and
 the program in comment:4. The attached program compiles without issue on
 GHC 8.0.2 and earlier, but panics on GHC HEAD (post–join points). The
 program in comment:4, however, compiles without issue on GHC 7.10.3 and
 earlier, but panics on GHC 8.0.1, 8.0.2, //and// HEAD.

 This isn't terribly surprising, as [https://github.com/ekmett/hybrid-
 vectors/issues/7#issue-133619382 others have reported] seeing this error
 on earlier versions of GHC, but AFAICT, no one was able to minimize the
 panic down to a single file until now. So the issue likely predates join
 points, but join points have made it easier to trigger in GHC HEAD.

 I'll see if I can figure out what commit lead to the panic surfacing in
 the program in comment:4.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13410#comment:5>
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