FYI panic "flatten_args wandered into deeper water than usual"
nicolas.frisby at gmail.com
Sun Jan 6 04:46:18 UTC 2019
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
> pprPanic, called at compiler/typecheck/TcFlatten.hs:1331:24 in
> 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
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