Disallow user_consent where experimental MSC3861 is enabled (#16127)

changelog.d/16127.bugfix

+User consent features cannot be enabled when using experimental MSC3861.

synapse/config/experimental.py

@@ -173,6 +173,13 @@ class MSC3861:
                 ("enable_registration",),
             )
 
+        # We only need to test the user consent version, as if it must be set if the user_consent section was present in the config
+        if root.consent.user_consent_version is not None:
+            raise ConfigError(
+                "User consent cannot be enabled when OAuth delegation is enabled",
+                ("user_consent",),
+            )
+
         if (
             root.oidc.oidc_enabled
             or root.saml2.saml2_enabled

tests/config/test_oauth_delegation.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import os
 from unittest.mock import Mock
 
 from synapse.config import ConfigError
@@ -167,6 +168,21 @@ class MSC3861OAuthDelegation(TestCase):
         with self.assertRaises(ConfigError):
             self.parse_config()
 
+    def test_user_consent_cannot_be_enabled(self) -> None:
+        tmpdir = self.mktemp()
+        os.mkdir(tmpdir)
+        self.config_dict["user_consent"] = {
+            "require_at_registration": True,
+            "version": "1",
+            "template_dir": tmpdir,
+            "server_notice_content": {
+                "msgtype": "m.text",
+                "body": "foo",
+            },
+        }
+        with self.assertRaises(ConfigError):
+            self.parse_config()
+
     def test_password_config_cannot_be_enabled(self) -> None:
         self.config_dict["password_config"] = {"enabled": True}
         with self.assertRaises(ConfigError):