Bug report: "AThing evaluated unexpectedly tcTyVar a_alF"

The following code:

{-# LANGUAGE GADTs, EmptyDataDecls, TypeFamilies, TypeOperators, DataKinds, FlexibleInstances #-}

{- Defines a C-like printf function using DataKinds extensions. -}

module Printf where

-- format string parameterized by a list of types
data Format (fmt :: [*]) where
  X :: Format '[]                   -- empty string, i.e. ""
  L :: a -> String -> Format '[]    -- string literal, e.g. "hello"
  S :: a -> Format '[String]        -- "%s"
  I :: Format a -> Format '[Int, a] -- "%d"

produces the following error:

GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Printf ( Printf.hs, interpreted )

Printf.hs:12:27:ghc: panic! (the 'impossible' happened)

(GHC version 7.10.3 for x86_64-unknown-linux):

AThing evaluated unexpectedly tcTyVar a_alF

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

when run using **ghci Printf.hs**

on Liunx Ubuntu 16.04 LTS 64-bit with Intel® Core™ i7-7500U CPU @ 2.70GHz