Not possible to build and install mypy with mypyc support, and run the tests against it.

mypy's built wheels includes two installable packages available after mypy is fully installed. They appear to not be practically usable:

• import mypy.test
• import mypyc.test

The testsuite depends on conftest.py and pyproject.toml from the root of the repository to run. Also, the test-data/ directory provides data the tests need. None of this data is distributed in the built wheels.

The fact that the packages are installed at all appears to be a bug. This is a side effect of #2784 which requested that the sdist (not the wheel) include the testsuite so that redistributors such as Debian can run the tests when packaging.

The intertwining of the testsuite as part of the mypy module alongside the requirement to use data from the sources makes it difficult to test mypy correctly. Allowing the tests to be detected by pytest means making the mypy/ directory importable and causes the tests to incorrectly run against the pure-python version, which means the compiled extensions do not get tested.

And mypy's own CI is working around this with editable installs: https://github.com/python/mypy/blob/9b9152484c6b1ba3934373ca0c7600f71392fb06/.github/workflows/test.yml#L137-L145

Also, the installation footprint of these unused files seems to be a couple of MB, which could be a nice optimization...

Start by reading .github/workflows/test.yml around the editable-install workaround, then inspect conftest.py, pyproject.toml, and test-data/. Determine how the built wheel and source tree expose mypy.test and mypyc.test. Done means the test suite can exercise compiled extensions through an installed build without shipping unusable test packages or required test data in the wheel.