I personally do not compost any poisonous or invasive weeds, however some members choose to do so with success per their admission. I feel there are too many other ingredients available to take such a chance. I found the link below and am sure there are numerous others if you "Google" a search. On that web page is does state:

"Poison hemlock must be removed. Dig it out, and do not allow this weed to go to seed. Wear gloves when handling it. Don't put it into the compost -dead stalks can remain poisonous for two or three seasons. Don't incinerate it."

As well, the page does offer outside links to other resources/information that may be of use in answering your question further. Have a great day, and please do exercise care if you are forced to have to handle Hemlock...

Blutranes

Here is a link that might be useful: Poisonous Weeds

Yeah, while I'm up for composting meat and such that you're "not supposed" to compost I think I'd shy away from something like this...
UNLESS I was doing something like Humanure where the pile was going to sit for 3 years...
If there wasn't a tremendous amount of the stuff I might put the poison hemlock down on the ground, put some greens over it, then some cardboard and maybe a metal pipe (to mark the divide) and build a pile over that. When the compost above was done I'd dig only to the pipe, then build another pile. I'd keep doing that for maybe a couple years by which point the stuff underneith would be effectively gone... Pull the pipe and compost above normally.

But composting poison, not sure I'd ever feel real good about that. Not that I'd be worried it'd get into my food but nervous I'd have a reaction to it while handling the compost...

Is "hemlock" different from "water hemlock"?

MMWR on "water hemlock"