TypedDict -> dict compatibility

Feature

Allow for TypedDicts (or a new type specialized for this purpose) to be compatible with dict when the receiver declares it is not going to mutate the dict.

Pitch

When using a TypedDict, it is common to eventually pass it to a function that takes an arbitrary dict or return it to the caller expecting the same. Currently this is rejected, and the reason makes sense. For example, this is correctly rejected:

class DefinitelyContainsStatus(TypedDict):
    status: str

def delete_status(d: dict):
    del d['status']

d: DefinitelyContainsStatus = {'status': 'ok'}
delete_status(d)
assert d['status'] == 'ok'

If it were permitted, a type checker wouldn't be able to know how the dict is going to be mutated, and then any other references that may exist would still expect it to conform to the TypedDict.

However, this presumably valid case is also rejected:

def foo() -> DefinitelyContainsStatus:
    return {'status': 'ok'}

f: dict[str, str] = foo()

# error: Incompatible types in assignment (expression has type "DefinitelyContainsStatus", variable has type "Dict[str, str]")  [assignment]

Here's a real-world example that has lead to the creation of this feature request. In Flask it is allowed to return a dict from a view function, which it then converts to JSON. This is particularly convenient in order to apply type checking to a JSON-based HTTP API. However, when the return type is declared to be some TypedDict, this is rejected:

class Example(TypedDict):
    status: str

@blueprint.route("/example")
def example() -> Example:
    return {"status": "ok"}

# error: Value of type variable "ft.RouteDecorator" of function cannot be "Callable[[str], Example]"  [type-var]

On the Flask side of things, they specifically require a real dict, so they cannot change the type to accept Mapping. It would be quite useful to be able to allow returning TypedDict from a function to a caller that accepts dict.

Es werden keine Dateien, Tests oder Einstiegspunkte genannt. Beginne damit, die Kompatibilitätsbeispiele für TypedDict und dict im Issue zu untersuchen, und verfolge anschließend die bestehenden Regeln des Typecheckers für Zuweisungs- und Callable-Kompatibilität. Als abgeschlossen gilt die Änderung, wenn sichere, nicht mutierende Verwendungen akzeptiert werden, während das Löschbeispiel weiterhin abgelehnt wird.