Protect Nursery against calls as sync ctx manager

tests/test_misuse.py

@@ -19,3 +19,11 @@ async def test_task_not_awaitable(arg):
     with pytest.raises(OSError):
         async with Nursery() as n:
             n.start_soon(arg)
+
+
+@pytest.mark.asyncio
+async def test_sync_ctx_manager():
+    """Calling nursery as a synchronous context manager"""
+    with pytest.raises(RuntimeError):
+        with Nursery():
+            pass

traio/nursery.py

@@ -92,6 +92,15 @@ async def __aexit__(self, exc_type, exc_val, exc_tb):
             self.cancel(exc_val)
         await self.join()
 
+    def __enter__(self):
+        """Protect against calling as regular context manager"""
+        raise RuntimeError(
+            "asynchronous context manager, use 'async with Nursery(...)'!"
+        )
+
+    def __exit__(self, *_):  # pragma: no cover
+        assert False, 'This should never be called'
+
     async def _timeout_handler(self):
         await asyncio.sleep(self.timeout)
         self.cancel(TimeoutError())