Room summary is only provided for lazy loading /syncs

[matrixbot]

This issue has been migrated from #15017.

---

As noted by @S7evinK here:

Also see matrix-org/complement#553 and matrix-org/matrix-spec#1325, where we came to the conclusion, for now, that it's not clear by the spec when this field should be added.
Looking at RoomSummary and the m.heroes, it states that this is required, if the room has neither m.room.name nor m.room.canonical_alias. So given a room can have neither (e.g. DMs), shouldn't Synapse include a RoomSummary, no matter if lazy loading or not?

Synapse only provides a room summary in /sync responses when lazy loading is enabled. However, the spec says that many of summary's fields (but not summary itself) are required under some conditions. eg. m.heroes is required if the room’s m.room.name or m.room.canonical_alias state events are unset or empty.

We should provide a room summary under the conditions mentioned by the spec and not just when lazy loading.