Panic in TcEvidence due to wrong role

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.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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