https://github.com/apple/swift/pull/20152 でなされた意図的なものなのですね。-swift-version 4.2 での互換性は保たれているはず。

[AST/Sema] Associate @autoclosure flag with parameter declaratio...

These changes aim to: Move @autoclosure flag from function type to parameter declaration Serialize @autoclosure as part of the parameter declaration Convert it from being a type attribute to be d...