[Haskell-beginners] What does 'impossible happened' mean?

[Baa]

Hello, List. I hit error:

    ghc: panic! (the 'impossible' happened)
      (GHC version 8.0.2 for x86_64-unknown-linux):
        initTc: unsolved constraints
      WC {wc_insol = [W] spec_a86e :: t_a86d[tau:1] (CHoleCan: spec)}
    
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

what does it mean and how to fix it?

PS. I only added template Haskell "call" in my test suite like

  $(blahBlah something)

The same call works fine in other modules and in GHCI. "stack build"
works too, but tests - no (last stack 1.6.1)

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Best regards, Paul