[GHC] #11681: ghc panic with TypeError

[GHC]

#11681: ghc panic with TypeError
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        Reporter:  inaki             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by aavogt):

 I can't reproduce the original problem with -rc2 either, but I get the
 original error message with this file:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 class C x where m :: x ()
 class B x

 instance B x => C x
 }}}

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
         fvProv falls into a hole {a1eE}
 }}}

 Interestingly the kind error is in there if we're missing
 FlexibleInstances

 {{{
 kind_error_fvprov.hs:12:17: error:
     ? Illegal instance declaration for ?C x?
         (All instance types must be of the form (T a1 ... an)
          where a1 ... an are *distinct type variables*,
          and each type variable appears at most once in the instance head.
          Use FlexibleInstances if you want to disable this.)
     ? In the instance declaration for ?C x?

 kind_error_fvprov.hs:12:19: error:
     ? Expected kind ?* -> *?, but ?x? has kind ?*?
     ? In the first argument of ?C?, namely ?x?
       In the instance declaration for ?C x?
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11681#comment:5>
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