"No skolem info"

On GHC 8.6.3, I am trying to typecheck the following code:

{-# LANGUAGE RankNTypes, TypeFamilies, PolyKinds, FunctionalDependencies #-}

import Data.Kind

class CatMonad (c :: k -> k -> Type) (m :: forall (x :: k) (y :: k). c x y -> Type -> Type) | c -> m where
  type Id c :: c x x

  xpure :: a -> m (Id c) a

boom :: forall (c :: k -> k -> Type) (m :: forall (x :: k) (y :: k). c x y -> Type -> Type) a. CatMonad c m => a -> m (Id c) a
boom = xpure

GHC panics with the following error message:

error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.3 for x86_64-unknown-linux):
	No skolem info:
  [k_a2vT[sk:0]]
  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

If you're thinking, "oh my god, why would you write code like that?!?!?!", the background is this Stack Overflow answer, and specifically, pining for overriding return and (>>=) using RebindableSyntax to be able to use do-notation with (category-)indexed monads.