Ah really? What is the purpose of that?

Not sure. Via https://stackoverflow.com/questions/19236117/what-numerical-... I found https://people.freebsd.org/~das/kahan86branch.pdf, but that's a more complex read than I'm prepared for right now ;)

That paper is about implementing branch cuts in a complex-valued function (e.g. complex square root), and doesn’t discuss real square roots.

In the context of that paper it seems to me that √(–0 – 0i) should be 0 – 0i and √(–0 + 0i) should be 0 + 0i, but under no circumstances should the result be –0 ± 0i, which is on the wrong branch.

The obvious extension to real-valued square root would be √(–0) == +0.