When someone invokes the build operation via the build_meta (via https://github.com/pypa/setuptools/blob/master/setuptools/build_meta.py#L170 - https://www.python.org/dev/peps/pep-0517/#build-sdist) the setup.py is no longer automatically packaged.

In case of python setup.py sdist what ensured that this will happen is https://github.com/python/cpython/blob/master/Lib/distutils/command/sdist.py#L250. Here distribution.script_name is set to setup.py when called via python setup.py sdist. This is not set though when calling it directly via build_meta so packaging setup.py is safely ignored.

Note the files to package is determined while running egg_info, and this is re-used across runs (by reading SOURCES.txt). Therefore this bug is hidden when:

• anyone beforehand calls python setup.py sdist,
• when there's a distutils extension that automatically pulls in setup.py (such as setuptools-scm),
• when some config value pulls in (one in MANIFEST.in, setup.cfg, setup.py).