As Guido suggested, I've created a new draft PEP for "User-defined Type Guards". Input is welcome.
Draft Proposal (in fork): https://github.com/erictraut/peps/blob/master/pep-9999.rst Draft Proposal (branch compare): https://github.com/python/peps/compare/master...erictraut:master
-- Eric Traut Contributor to Pyright and Pylance Microsoft Corp.