Disable `m.3pid_changes` capability when MSC3861 is enabled. (#16134)

changelog.d/16127.bugfix

@@ -1 +1 @@
-User consent features cannot be enabled when using experimental MSC3861.
+User constent and 3-PID changes capability cannot be enabled when using experimental [MSC3861](https://github.com/matrix-org/matrix-spec-proposals/pull/3861) support.

changelog.d/16134.bugfix

@@ -0,0 +1 @@
+User constent and 3-PID changes capability cannot be enabled when using experimental [MSC3861](https://github.com/matrix-org/matrix-spec-proposals/pull/3861) support.

synapse/config/experimental.py

@@ -223,6 +223,12 @@ class MSC3861:
                 ("session_lifetime",),
             )
 
+        if root.registration.enable_3pid_changes:
+            raise ConfigError(
+                "enable_3pid_changes cannot be enabled when OAuth delegation is enabled",
+                ("enable_3pid_changes",),
+            )
+
 
 @attr.s(auto_attribs=True, frozen=True, slots=True)
 class MSC3866Config:

synapse/config/registration.py

@@ -133,7 +133,16 @@ class RegistrationConfig(Config):
 
         self.enable_set_displayname = config.get("enable_set_displayname", True)
         self.enable_set_avatar_url = config.get("enable_set_avatar_url", True)
-        self.enable_3pid_changes = config.get("enable_3pid_changes", True)
+
+        # The default value of enable_3pid_changes is True, unless msc3861 is enabled.
+        msc3861_enabled = (
+            (config.get("experimental_features") or {})
+            .get("msc3861", {})
+            .get("enabled", False)
+        )
+        self.enable_3pid_changes = config.get(
+            "enable_3pid_changes", not msc3861_enabled
+        )
 
         self.disable_msisdn_registration = config.get(
             "disable_msisdn_registration", False

tests/config/test_oauth_delegation.py

@@ -271,3 +271,8 @@ class MSC3861OAuthDelegation(TestCase):
         self.config_dict["session_lifetime"] = "24h"
         with self.assertRaises(ConfigError):
             self.parse_config()
+
+    def test_enable_3pid_changes_cannot_be_enabled(self) -> None:
+        self.config_dict["enable_3pid_changes"] = True
+        with self.assertRaises(ConfigError):
+            self.parse_config()