On May 30, 2017, at 6:34 AM, Nick Coghlan <ncoghlan@gmail.com> wrote:

Note that I'm also fine with pip as a project saying that it will only
ship support for the build-backend interface once the source filtering
interface is also defined and implemented.

As an addendum, this seems entirely silly to me as well. The packaging landscape is already littered with unimplemented PEPs that confuse people, adding another one seems like a waste of everyone’s time, and if this one is dependent on another one, it seems silly not to define them together.