https://github.com/apple/swift-evolution/blob/main/commonly_proposed.md#control-flow-closures-optional-binding-and-error-handling これか。確かによほどじゃないと採用されないリストにあるね。

swift-evolution/commonly_proposed.md at main · apple/swift-evolution

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - swift-evolution/commonly_proposed.md at main · apple/swift-evolution