Thanks for pointing me to that library @Nevin!
I'm still pretty new to the community, so I haven't been following along what everyone has been saying until recently. However, I did notice that SE-0246: Generic Math Functions was accepted, which introduced the Real protocol as standard in Swift without name spacing it in Math. So it seems there could be a possibility of adding Complex type as part of the standard library.
However, for what I'll be doing (differentiating complex numbers), I don't have a strong opinion on where it will exist, and the decision about where it will exist can be made when it's brought up in Swift Evolution when/if complex numbers are added. So I think the topic of where it will belong can be discussed later in the future.