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

首先定位 typing 一致性测试套件，以及其中针对 TypeVar 默认值和默认参数的现有测试。添加所提议的 Getter 示例，并包含预期的定义和赋值诊断，然后比较所列各类型检查器的结果，以明确一致性预期。