add test to check for cogent3#1844

tests/test_app/test_plugins.py

@@ -258,3 +258,20 @@ def main(self, data: int) -> int:
     assert app_with_default_addapp(5) == 42
     app_with_custom_addapp = get_app("AppWithDefault", app=AddApp(10))
     assert app_with_custom_addapp(5) == 15
+
+def test_app_help_from_function(mock_extension_manager):
+    """_make_apphelp_docstring on a decorated function should return help"""
+
+    @define_app
+    def square(val: int) -> int:
+        """app that returns the square of the input value"""
+        return val*val
+
+
+    mock_extension_manager([create_extension(square, module_name="module1")])
+
+    assert square.__name__ in cogent3.app.get_app_manager().names()
+    got = _make_apphelp_docstring(square)
+
+    assert "Options" in got  # test help is rendered
+    assert "square_app" in got  # test the help is for the correct app