Raise a runtime error if run_async is called without real ContextVars

Werkzeug offers a ContextVar replacement for Python < 3.7, however it
doesn't work across asyncio tasks, hence it makes sense to error out
rather than find there are odd bugs.

Note the docs build requires the latest (dev) Werkzeug due to this
change (to import ContextVar from werkzeug.local).

src/flask/helpers.py

@@ -7,6 +7,7 @@ from threading import RLock
 
 import werkzeug.utils
 from werkzeug.exceptions import NotFound
+from werkzeug.local import ContextVar
 from werkzeug.routing import BuildError
 from werkzeug.urls import url_quote
 
@@ -741,6 +742,11 @@ def run_async(func):
             "Install Flask with the 'async' extra in order to use async views."
         )
 
+    if ContextVar.__module__ == "werkzeug.local":
+        raise RuntimeError(
+            "async cannot be used with this combination of Python & Greenlet versions"
+        )
+
     @wraps(func)
     def outer(*args, **kwargs):
         """This function grabs the current context for the inner function.

tests/test_async.py

@@ -1,10 +1,12 @@
 import asyncio
+import sys
 
 import pytest
 
 from flask import abort
 from flask import Flask
 from flask import request
+from flask.helpers import run_async
 
 
 @pytest.fixture(name="async_app")
@@ -23,6 +25,7 @@ def _async_app():
     return app
 
 
+@pytest.mark.skipif(sys.version_info < (3, 7), reason="requires Python >= 3.7")
 def test_async_request_context(async_app):
     test_client = async_app.test_client()
     response = test_client.get("/")
@@ -31,3 +34,9 @@ def test_async_request_context(async_app):
     assert b"POST" in response.get_data()
     response = test_client.get("/error")
     assert response.status_code == 412
+
+
+@pytest.mark.skipif(sys.version_info >= (3, 7), reason="should only raise Python < 3.7")
+def test_async_runtime_error():
+    with pytest.raises(RuntimeError):
+        run_async(None)

tox.ini

@@ -25,5 +25,8 @@ skip_install = true
 commands = pre-commit run --all-files --show-diff-on-failure
 
 [testenv:docs]
-deps = -r requirements/docs.txt
+deps =
+    -r requirements/docs.txt
+
+    https://github.com/pallets/werkzeug/archive/master.tar.gz
 commands = sphinx-build -W -b html -d {envtmpdir}/doctrees docs {envtmpdir}/html