My example is different from the one shown in #3851 in that it also does not work in GHC 6.10 (which even panics instead of giving the error 6.12 gives) and in that it uses a data family, not a type family.
Whether or not it is the same "awkward interaction" that causes this behaviour is beyond my expertise.
Cheers,
Sebastian
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Underestimating the novelty of the future is a time-honored tradition.