warn for async gen functions

Author: graingert

src/_pytest/python.py

@@ -152,7 +152,9 @@ def pytest_configure(config):
 @hookimpl(trylast=True)
 def pytest_pyfunc_call(pyfuncitem):
     testfunction = pyfuncitem.obj
-    if iscoroutinefunction(testfunction):
+    if iscoroutinefunction(testfunction) or (
+        sys.version_info >= (3, 6) and inspect.isasyncgenfunction(testfunction)
+    ):
         msg = "async def functions are not natively supported and have been skipped.\n"
         msg += "You need to install a suitable plugin for your async framework, for example:\n"
         msg += "  - pytest-asyncio\n"

testing/acceptance_test.py

@@ -1227,11 +1227,11 @@ async def test_2():
         [
             "test_async.py::test_1",
             "test_async.py::test_2",
-            "*Coroutine functions are not natively supported*",
+            "*async def functions are not natively supported*",
             "*2 skipped, 2 warnings in*",
         ]
     )
     # ensure our warning message appears only once
     assert (
-        result.stdout.str().count("Coroutine functions are not natively supported") == 1
+        result.stdout.str().count("async def functions are not natively supported") == 1
     )