Panic! "No skolem info" using deriving-via
Create a file with the following contents and load it into GHCi:
{-# LANGUAGE DerivingVia #-}
import Data.Functor.Compose
newtype ReaderIO env a = ReaderIO (env -> IO a)
deriving (Functor, Applicative) via (Compose ((->) a) IO)
Result:
GHCi, version 8.6.5: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( example.hs, interpreted )
example.hs:6:15: error:ghc: panic! (the 'impossible' happened)
(GHC version 8.6.5 for x86_64-unknown-linux):
No skolem info:
[a_a1FF]
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in ghc:TcErrors
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
Note that this code shouldn't actually compile (on the last line I accidentally wrote a
instead of env
).