spec/conformance: function with TypeVar default and default argument used in context.

I want to suggest adding the following test to the conformance test suite:

class Getter[T]:
    def get[S=None](self, default: S = None) -> T | S: ...  # OK

def test(arg: Getter[str]) -> None:
    result: str = arg.get()  # type: ignore

• Since the default argument value is assignable to the default type var type, def get is well-defined
• arg.get() implicitly uses None of type None as the argument. Therefore, we have an implicit constraint S <: None when .get is called without explicit default.

Therefore, result: str = arg.get() should raise an assignment type error (str | None not assignalbe to str).

Currently, only pyright succeeds on this test, and type checker results diverge.

1. pyright: no errors (true negative on def get, true positive on result) (pyright playground)
2. mypy: false positive on def get, false negative on result.
3. ty: false positive on def get, true positive on result. (playground)
4. pyrefly: false positive on def get, false negative on result (playground)
5. zuban: false postive on def get, false negative on result.

Related Issues

• https://github.com/python/mypy/issues/3737 (one of the most upvotes issues on mypy)
• https://github.com/astral-sh/ty/issues/592

Start by locating the typing conformance test suite and its existing tests for TypeVar defaults and default arguments. Add the proposed Getter example with the expected definition and assignment diagnostics, then compare the results across the listed type checkers so the conformance expectation is explicit.