Typeguard for TypedDict keys

Feature

I don't know enough to say whether this is a mypy request or a typing module request... but I find myself doing this pattern to avoid the error TypedDict key must be a string literal; expected one of ("a", "b") [misc]

class MyDict(TypedDict, total=False):
    a: int
    b: str
    ...

MyDictKeys = Union[Literal['a'], Literal['b']]

def is_mydict_key(key: str) -> TypeGuard[WidgetOptionKeys]:
    return key in MyDictKeys.__annotations__

Pitch

not sure how it would be implemented (or if it already is!) ... but it would be nice to have a way to type narrow a key without having to duplicate all of the keys in a TypedDict in a separate Union of Literals

Start by determining whether this belongs in mypy or the typing module, then investigate how TypedDict key checking and TypeGuard interact. Compare the current duplicate Literal-union workaround with possible key narrowing approaches. Done means a supported way to narrow a TypedDict key without separately duplicating its keys.