On Jul 16, 2018, at 5:22 AM, Paul Moore <p.f.moore@gmail.com> wrote:

1. If [build-system] is present but requires is missing, raise an error.
2. If [build-system] is missing, they can take one of the following
two approaches:
  a) Act as if pyproject.toml is missing altogether
  b) Act as if [build-system] is present, with a requires value of
["setuptools", "wheel"]

Whether tools act differently in cases 2a and 2b is tool-dependent
(for pip, we would isolate in case 2b but not in case 2a) which is why
the choice is left to individual tools. That makes the
"Thomas/Nathaniel" debate into a tool implementation choice, and both
of the options are allowable from the perspective of the PEP.

This sounds fine to me, and I prefer a 2b approach.