The following program results in a ghc panic:

{-# LANGUAGE DerivingVia #-}

import Data.Functor.Const (Const(..))
import Data.Functor.Classes

newtype FlipConst a b = FlipConst b
  deriving (Show1, Eq1) via (Const b)

Here is the output of the compile:

test/Spec/Contravariant.hs:52:13: error:
    • No instance for (Show b)
        arising from the 'deriving' clause of a data type declaration
      Possible fix:
        add (Show b) to the context of
          the deriving clause for ‘Show1 (FlipConst a)’
        or use a standalone 'deriving instance' declaration,
             so you can specify the instance context yourself
    • When deriving the instance for (Show1 (FlipConst a))
   |
52 |   deriving (Show1, Eq1) via (Const b)
   |             ^^^^^

test/Spec/Contravariant.hs:52:13: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.3 for x86_64-unknown-linux):
	No skolem info:
  [b_a3eO]
  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

I'm using GHC 8.6.3