"unresolved constraints" when import and main missing

I gut a weird error because I forgot an import and the main method. (the commented lines in the code below)

GHCi told me to post a bug about this, so... :-)

Steps to reproduce:

> ghci
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Prelude> :l bug.hs
[1 of 1] Compiling Main             ( bug.hs, interpreted )
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for x86_64-unknown-mingw32):
        initTc: unsolved constraints
  WC {wc_insol = [W] printf_a2MG :: t_a2MF[tau:1] (CHoleCan: printf)}

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

File bug.hs:

module Main where

import Test.Hspec
import Test.QuickCheck

-- import Text.Printf

testMovie :: Int -> Int -> Double -> Int -> Spec
testMovie card ticket perc s = 
    it (printf "should return Movie for card: %d, ticket: %d perc %s " card ticket (show perc)) $
        movie card ticket perc `shouldBe` s

movie :: Int -> Int -> Double -> Int
movie card ticket perc = 4

-- main :: IO ()
-- main = undefined

Expected behavior: Useful error message.

Actual behavior: "panic" error message.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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