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