DeriveAnyClass + deriving Bifunctor causes GHC panic

[erantapaa]

Attempting the derive Bifunctor with DeriveAnyClass caused this message from GHC:

$ ghc Foo.hs
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )
Var/Type length mismatch:
  [a_an7, b_an8]
  []
Var/Type length mismatch:
  [a_an7, b_an8]
  []
Var/Type length mismatch:
  [a_an7, b_an8]
  []

Foo.hs:8:13:ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for x86_64-apple-darwin):
	tcTyVarDetails a_an7

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

The code:

{-# LANGUAGE DeriveAnyClass #-}

module Foo where

import Data.Bifunctor

data Blah a b = A a | B b
  deriving (Bifunctor)

[thomie]

Reproducable with HEAD (ghc-7.11.20151002).

[dreixel]

Thanks for reporting, but this is already reported as #9968.