[GHC] #12212: GHC 8.0.1 crash

[GHC]

#12212: GHC 8.0.1 crash
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        Reporter:  dibblego          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  9160, 10602       |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Indeed something smells a bit fishy here. Looking at rule in question, we
 see that the coercion variable which is supposed unbound (`cobox_s1TP`) is
 trivially provided by `Refl`.
 {{{
       [LclId,
        Arity=10,
        Str=DmdType,
        RULES: "SPEC t @ a @ 'DQuantity @ 'DQuantity @ d @ d' @ 'DQuantity
 @ 'DQuantity @ d @ d'" [ALWAYS]
                   forall (cobox_s1TP
                             :: ((d_a1Fl * d'_a1Fm) :: Dimension)
                                GHC.Prim.~#
                                ((d_a1Fl * d'_a1Fm) :: Dimension))
                           ...
 }}}

 It looks like `specialise/Rules.hs:matchN` has some logic for handling
 unbound type variables which will fill in with `Any`; however this is a
 coercion variable, so we instead fail. I wonder if coercion variables also
 need some special treatment.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12212#comment:3>
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