fix return types of athrow and aclose, error when async generator returns a value

Author: JelleZijlstra

mypy/checker.py

@@ -1628,7 +1628,7 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                 if isinstance(typ, AnyType):
                     return
 
-                if self.is_unusable_type(return_type):
+                if self.is_unusable_type(return_type) or defn.is_async_generator:
                     # Lambdas are allowed to have a unusable returns.
                     # Functions returning a value of type None are allowed to have a Void return.
                     if isinstance(self.scope.top_function(), FuncExpr) or isinstance(typ, NoneTyp):

test-data/unit/check-async-await.test

@@ -379,7 +379,7 @@ async def h() -> None:
     g = gen()
     v = await g.asend(1)
     reveal_type(v)  # E: Revealed type is 'builtins.str*'
-    await g.athrow(BaseException)
+    reveal_type(await g.athrow(BaseException))  # E: Revealed type is 'builtins.str*'
 
 [builtins fixtures/dict.pyi]
 
@@ -419,7 +419,7 @@ from mypy_extensions import AsyncGenerator
 
 async def gen() -> AsyncGenerator[int, None]:
     yield 1
-    return 42
+    return 42  # E: No return value expected
 
 [builtins fixtures/dict.pyi]
 

test-data/unit/lib-stub/typing.pyi

@@ -66,10 +66,10 @@ class AsyncGenerator(AsyncIterator[T], Generic[T, U]):
     def asend(self, value: U) -> Awaitable[T]: pass
 
     @abstractmethod
-    def athrow(self, typ: Any, val: Any=None, tb: Any=None) -> Awaitable[None]: pass
+    def athrow(self, typ: Any, val: Any=None, tb: Any=None) -> Awaitable[T]: pass
 
     @abstractmethod
-    def aclose(self) -> Awaitable[None]: pass
+    def aclose(self) -> Awaitable[T]: pass
 
     @abstractmethod
     def __aiter__(self) -> 'AsyncGenerator[T, U]': pass