Sendable conformances for source compatibility.
If conformance lookup always prefers the conformance from the defining module,
libraries introducing unavailable Sendable conformances can break source in
clients that declare retroactive Sendable conformances. Instead, still prefer
the available conformance, and always diagnose the client conformance as
redundant (as a warning). Then, when the retroactive conformance is removed,
the errors will surface, so the programmer has to take explicit action to
experience the source break.
defining module, and diagnose redundant Sendable conformances.
We still allow re-stating inherited unchecked Sendable conformances in
subclasses because inherited Sendable conformances are surprising when
they opt out of static checking. Otherwise, warning on redundant Sendable
conformances nudges programmers toward cleaning up unnecessary retroactive
Sendable conformances over time as libraries incrementally add the
conformances directly.
