[GHC] #10489: Panic in TcEvidence due to wrong role

[GHC]

#10489: Panic in TcEvidence due to wrong role
-------------------------------------+-------------------------------------
              Reporter:  goldfire    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  highest     |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 This single line tickles it:

 {{{
 convert d = let d' = case d of '0' -> '!' in d'
 }}}

 produces

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.11.20150604 for x86_64-apple-darwin):
         ASSERT failed!
   file compiler/typecheck/TcEvidence.hs line 600 Sym cobox_af8M

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 The failing assertion is about roles. Will look into this tomorrow.

 Labeling this "highest" because it's a very silly regression.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10489>
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