"panic: No skolem info" with invalid instance signature

Summary

The following module (Bug.hs) causes a panic when compiling:

{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

type family F a

class Class a where
  f :: b -> a

instance Class a where
  f :: (F b ~ a) => a
  f = undefined

Bug.hs:12:8: error:ghc: panic! (the 'impossible' happened)
  (GHC version 9.2.0.20210422:
	No skolem info:
  [a_auH[ssk:1]]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Tc/Errors.hs:2873:17 in ghc:GHC.Tc.Errors

The panic seems to be specific to this combination of a type family, an equality constraint, and the invalid signature in the instance. Changing any of these makes GHC fail with a proper error message.

Steps to reproduce

Compile the above module.

Expected behavior

Fail with a proper error message.

Environment

• GHC version used: 8.10.4, 9.0.1, 9.2.0.20210422 (via ghcup)

Optional:

• Operating System: Linux 5.10.31
• System Architecture: x86_64