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

Inizia individuando la suite di test di conformità di typing e i test esistenti per i valori predefiniti di TypeVar e gli argomenti predefiniti. Aggiungi l'esempio Getter proposto con la definizione prevista e le diagnostiche di assegnazione previste, quindi confronta i risultati tra i type checker elencati in modo che l'aspettativa di conformità sia esplicita.