The return type of a generator function (any function containing yield) is a typing.Generator (or typing.AsyncGenerator if its an async def); this type must be assignable to the annotated return type.

The type of a yield expression inside the function is the second type argument of the annotated Generator or AsyncGenerator return type, for example:

from typing import Generator

def unannotated():
    x = yield 10
    reveal_type(x)  # revealed: Unknown

def default_specialized() -> Generator:
    x = yield 10
    # The `_SendT_contra` typevar on `Generator` has `default=None`
    reveal_type(x)  # revealed: None

def partially_specialized() -> Generator[int]:
    x = yield 10
    # The first type parameter of `Generator` is the yielded type (so `10` here must be assignable to it)
    # The second type parameter is the "send" type, which is still default-specialized to `None` here:
    reveal_type(x)  # revealed: None

def fully_specialized() -> Generator[int, str]:
    x = yield 10
    reveal_type(x)  # revealed: str

def iterator() -> Iterator[int]:
    x = yield 10
    # The `Iterator` return type here is a supertype of `Generator`.
    # It has no "send" type, nor a send method, so this should always be `None`
    reveal_type(x)  # revealed: None

All those examples would look the same for an async def, except the return type would need to be AsyncGenerator, not Generator.

Support yield #1718

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Support `yield` #1718