I don't have a clang-tidy setup yet (hard to figure out on Windows), but I think this going to fire an else-after-return warning. I'll fix this and and also add an unreachable assert at the end of this function.

I don't think this will work, because the same multi-function could be used by multiple threads that use different pivot axes.
There could be three different static multi-functions I suppose, which would still avoid allocating a new one every time.

Looks like this isn't using the mask, the static functions above should use that to avoid doing unnecessary calculations.