This code works fine in GHC 8.0.1 and 8.0.2:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Foo where

import Data.Kind

data Foo (a :: Type) (b :: Type) where
  MkFoo :: (a ~ Int, b ~ Char) => Foo a b

data family Sing (a :: k)
data SFoo (z :: Foo a b) where
  SMkFoo :: SFoo MkFoo

But in GHC 8.2 and HEAD, it panics:

$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170622: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Foo              ( Bug.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.0.20170622 for x86_64-unknown-linux):
	typeIsTypeable(Coercion)