only try to LL members after a non-gappy incr sync if the timeline has events

changelog.d/3936.bugfix

@@ -0,0 +1 @@
+Fix out-of-bounds error when LLing yourself

synapse/handlers/sync.py

@@ -713,10 +713,6 @@ def compute_state_delta(self, room_id, batch, sync_config, since_token, now_toke
                     )
                 ]
 
-                # always make sure we LL ourselves so we know we're in the room
-                # (if we are), to fix https://github.com/vector-im/riot-web/issues/7209
-                types.append((EventTypes.Member, sync_config.user.to_string()))
-
                 # only apply the filtering to room members
                 filtered_types = [EventTypes.Member]
 
@@ -726,6 +722,13 @@ def compute_state_delta(self, room_id, batch, sync_config, since_token, now_toke
             }
 
             if full_state:
+                if lazy_load_members:
+                    # always make sure we LL ourselves so we know we're in the room
+                    # (if we are) to fix https://github.com/vector-im/riot-web/issues/7209
+                    # We only need apply this on full state syncs given we disabled
+                    # LL for incr syncs in #3840.
+                    types.append((EventTypes.Member, sync_config.user.to_string()))
+
                 if batch:
                     current_state_ids = yield self.store.get_state_ids_for_event(
                         batch.events[-1].event_id, types=types,
@@ -794,7 +797,7 @@ def compute_state_delta(self, room_id, batch, sync_config, since_token, now_toke
             else:
                 state_ids = {}
                 if lazy_load_members:
-                    if types:
+                    if types and batch.events:
                         # We're returning an incremental sync, with no
                         # "gap" since the previous sync, so normally there would be
                         # no state to return.