変更を見ると

Move @autoclosure flag from function type to parameter declaration

って書かれてるから、その理解で合ってそうですね。 https://github.com/apple/swift/pull/20152

[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...