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

@@ -1207,3 +1207,31 @@ async def test_2():
     assert (
         result.stdout.str().count("async def functions are not natively supported") == 1
     )
+
+
+@pytest.mark.filterwarnings("default")
+@pytest.mark.skipif(
+    sys.version_info < (3, 6), reason="async gen syntax available in Python 3.6+"
+)
+def test_warn_on_async_gen_function(testdir):
+    testdir.makepyfile(
+        test_async="""
+        async def test_1():
+            yield
+        async def test_2():
+            yield
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "test_async.py::test_1",
+            "test_async.py::test_2",
+            "*async def functions are not natively supported*",
+            "*2 skipped, 2 warnings in*",
+        ]
+    )
+    # ensure our warning message appears only once
+    assert (
+        result.stdout.str().count("async def functions are not natively supported") == 1
+    )