While developing a typechecker plugin, I experienced this error message.
ghc: panic! (the 'impossible' happened) (GHC version 8.6.2 for x86_64-unknown-linux): flatten_args wandered into deeper water than usual Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcFlatten.hs:1331:24 in ghc:TcFlatten
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
I'm sending this email just in case a dev is curious how this panic arises or if another plugin developer sees the same thing -- the error message is not very informative, and the corresponding GHC source code is a bit obtuse. Based on the relevant GHC source code and my recent plugin edits, I was able to guess correctly that this was due to me generating an extra sort argument to a promoted datacon. Specifically, I was incorrectly using
mkTyConApp (promoteDataCon _) [typeKind k,k,...]
instead of just
mkTyConApp (promoteDataCon _) [k,...]
Thanks! HTH. -Nick