I can confirm that your fix makes ForwardDiff work (and the value is sane).
What’s your opinion on the whether I need to put FullSpecialize vs NoSpecialize vs nothing? Which is most likely to help AD packages succeed?
ReverseDiff, TaylorDiff, Enzyme, and Zygote all fail for different reasons. I’ll try and notify various places about… 5 different issues or so. (I haven’t tried Enzyme except through DifferentiationInterface.)