AsyncGenerator: remove redundant __new__, add test case for instantiation

Author: JelleZijlstra

src/test_typing.py

@@ -1785,7 +1785,12 @@ def g(): yield 0
 
     @skipUnless(PY36, 'Python 3.6 required')
     def test_subclassing_async_generator(self):
-        class G(typing.AsyncGenerator[int, int]): ...
+        class G(typing.AsyncGenerator[int, int]):
+            def asend(self, value):
+                pass
+            def athrow(self, typ, val=None, tb=None):
+                pass
+
         ns = {}
         exec('async def g(): yield 0', globals(), ns)
         g = ns['g']
@@ -1795,6 +1800,13 @@ class G(typing.AsyncGenerator[int, int]): ...
         self.assertIsSubclass(G, collections.AsyncIterable)
         self.assertNotIsSubclass(type(g), G)
 
+        instance = G()
+        self.assertIsInstance(instance, typing.AsyncGenerator)
+        self.assertIsInstance(instance, typing.AsyncIterable)
+        self.assertIsInstance(instance, collections.AsyncGenerator)
+        self.assertIsInstance(instance, collections.AsyncIterable)
+        self.assertNotIsInstance(type(g), G)
+
     def test_subclassing_subclasshook(self):
 
         class Base(typing.Iterable):

src/typing.py

@@ -1867,12 +1867,6 @@ class AsyncGenerator(AsyncIterator[T_co], Generic[T_co, T_contra],
                          extra=_AG_base):
         __slots__ = ()
 
-        def __new__(cls, *args, **kwds):
-            if _geqv(cls, AsyncGenerator):
-                raise TypeError("Type AsyncGenerator cannot be instantiated; "
-                                "create a subclass instead")
-            return _generic_new(_AG_base, cls, *args, **kwds)
-
     __all__.append('AsyncGenerator')