panic initTc: unsolved constraints

I'm trying to play with some piece of code in GHCi. Sorry if it's a duplicate bug.\\

{-# LANGUAGE TemplateHaskell #-}

module Testbar where

printf :: String -> String
printf s = gen (parse s)

$(printf "Error: %s at line %d")

Prelude> :l testbar
[1 of 1] Compiling Testbar          ( testbar.hs, interpreted )
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for i386-unknown-mingw32):
        initTc: unsolved constraints
  WC {wc_insol =
        [W] gen_a10Q :: t_a10P[tau:1] (CHoleCan: gen)
        [W] parse_a10Z :: t_a10Y[tau:1] (CHoleCan: parse)}

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