Since we've already severely limited Sendable's abilities by making it a "marker" protocol, the biggest action-at-a-distance hazard I see with implicit conformances to it is name lookup. If we also say that extension Sendable extensions are not allowed to declare members, that should contain that effect, so that modifying a type's layout does not cause members to flit in and out of existence everywhere else. At that point, I think most of the consequence Sendable conformance appearing or disappearing ought to be contained to first-order "does not conform" errors in call sites that require sendability.
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