Check if the localpart is reserved for guests earlier in the registration flow (#7625)

This is so the user is warned about the username not being valid as soon as possible, rather than only once they've finished UIA.

changelog.d/7625.misc

@@ -0,0 +1 @@
+Check if the localpart of a Matrix ID is reserved for guest users earlier in the registration flow, as well as when responding to requests to `/register/available`.

synapse/handlers/register.py

@@ -128,6 +128,15 @@ class RegistrationHandler(BaseHandler):
                     errcode=Codes.FORBIDDEN,
                 )
 
+        if guest_access_token is None:
+            try:
+                int(localpart)
+                raise SynapseError(
+                    400, "Numeric user IDs are reserved for guest users."
+                )
+            except ValueError:
+                pass
+
     @defer.inlineCallbacks
     def register_user(
         self,
@@ -170,15 +179,6 @@ class RegistrationHandler(BaseHandler):
 
             was_guest = guest_access_token is not None
 
-            if not was_guest:
-                try:
-                    int(localpart)
-                    raise SynapseError(
-                        400, "Numeric user IDs are reserved for guest users."
-                    )
-                except ValueError:
-                    pass
-
             user = UserID(localpart, self.hs.hostname)
             user_id = user.to_string()