ghc: panic! (the 'impossible' happened)

GHC asked me nicely to report this compile-time panic / crash.

Minimal code:

{-# LANGUAGE Rank2Types #-}

module Foo where
import System.Random.MWC (GenST)
run :: (forall s . GenST s) -> Int
run = 0

Output:

$ ghc Foo.hs 
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.1 for x86_64-unknown-linux):
	tcTyVarDetails
    ( s{tv a10F} [tv] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )

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

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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