Definition Order Matters For `types.coroutine` Decorator

[rmorshea]

This fails mypy --strict (v0.782):

from types import coroutine
from typing import Any, Iterator

async def my_async_func() -> None:
    await my_coro_func()

@coroutine
def my_coro_func() -> Iterator[None]:
    yield None

but this passes:

from types import coroutine
from typing import Any, Iterator

@coroutine
def my_coro_func() -> Iterator[None]:
    yield None

async def my_async_func() -> None:
    await my_coro_func()

MyPy playground links:

• fail: https://mypy-play.net/?mypy=latest&python=3.8&flags=strict&gist=06aa146dfa9d86ca94b707cfa2367f27
• pass: https://mypy-play.net/?mypy=latest&python=3.8&flags=strict&gist=df2b842045c756988b171d6ebb9b3b6c

[JukkaL]

The problem seems to be that the correct type gets inferred too late, during type checking (in https://github.com/python/mypy/blob/master/mypy/checker.py):

                # Fix the type if decorated with `@types.coroutine` or `@asyncio.coroutine`.
                if defn.is_awaitable_coroutine:
                    # Update the return type to AwaitableGenerator.
                    # (This doesn't exist in typing.py, only in typing.pyi.)
                    t = typ.ret_type
                    c = defn.is_coroutine
                    ty = self.get_generator_yield_type(t, c)
                    tc = self.get_generator_receive_type(t, c)
                    if c:
                        tr = self.get_coroutine_return_type(t)
                    else:
                        tr = self.get_generator_return_type(t, c)
                    ret_type = self.named_generic_type('typing.AwaitableGenerator',
                                                       [ty, tc, tr, t])
                    typ = typ.copy_modified(ret_type=ret_type)
                    defn.type = typ

It we'd do this during semantic analysis, the order of definitions should not matter.