test_except_star does not cover except* handlers raising a non-Exception BaseException #160659

Lib/test/test_except_star.py has no test where an except* handler raises a BaseException that isn't an Exception (for example KeyboardInterrupt, SystemExit, or a custom BaseException subclass). Every handler in TestExceptStarRaise and TestExceptStarRaiseFrom raises an Exception subclass (TypeError or SyntaxError), and no other except* handler in Lib/test raises a BaseException-only type, directly or indirectly.

CPython already behaves correctly here: the raised exception propagates on its own, or is combined with any other raised exceptions and the unmatched remainder into a BaseExceptionGroup. But because nothing tests this, other implementations that run CPython's test suite aren't told when they get it wrong. PyPy passes all of test_except_star.py, yet raises an internal AssertionError in exactly this case, and also builds an ExceptionGroup where a BaseExceptionGroup is needed. See pypy/pypy#5689. The bug was found through a test in the exceptiongroup backport (agronholm/exceptiongroup#163), not through CPython's tests.

I'll send a test-only PR that adds tests for this case to TestExceptStarRaise.

Linked PRs

• gh-160659: Add tests for except* handlers that raise a BaseException #160660