Based on the discussion, I think the real danger of subclassing is unexpected behaviour - in other words, overriding methods. There doesn't seem to be a need to penalise subclasses which just add properties and extra methods based on them.
I'd be in favour of keeping current behaviour for classes, but having to mark methods explicitly as overrideable, virtual, open, or whatever semantics we decide upon. This seems like the safest syntax.
I'd also support a grouping mechanism for marking methods in this way, similar to extensions' current auto-annotation for ‘public’.
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Hello Swift community,
The second review of "SE-0117: Default classes to be non-subclassable publicly" begins now and runs through July 22. The proposal is available here:
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