Swift 6.4 ships async defer, noncopyable associated types, and source-level warning control

Sources: primary, changelog
Summary: The Swift 6.4 release is tagged swift-6.4.0-RELEASE and was published 2026-09-15. Its language changes include async defer, noncopyable associated types, and source-level control of warnings. Unstructured throwing Task initializers now use typed throws and warn when a throwing operation's result is neither stored nor ignored, and forward references to local variables inside closures that the type checker previously accepted are now rejected.
Why it matters: The concurrency changes alter existing code rather than only adding surface, so both are upgrade work rather than opt-in features.
Follow-up: The release body on the tag is empty, so the CHANGELOG at the tag is the readable primary for any Swift release check.