Property Observers (willset/didset) — Proposal

Issue: #416 Status: Proposal (not committed to roadmap) Extracted from: feature_roadmap.md

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Swift-inspired property observers for side effects on auto-properties. This is a proposal — not yet part of the committed language design.

Proposed Syntax

class Character:
    property health: int
        didset:
            print(f"health changed from {oldvalue} to {self.health}")
        willset(new_value):
            assert new_value >= 0

Open Questions

• Does this add enough value over converting to a function-style property with custom getter/setter?
• Should oldvalue be a contextual keyword or an explicit parameter?
• How does this interact with @override properties?
• Is this orthogonal to events, or does it overlap?

See #416 for ongoing discussion.