Panic in TcEvidence due to wrong role
This single line tickles it:
convert d = let d' = case d of '0' -> '!' in d'
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.