お、 @dynamicCallable の PR が作り直されていて [WIP] も外れてる。 https://github.com/apple/swift/pull/20305

Implement @dynamicCallable. by dan-zheng · Pull Request #20305 ...

This PR introduces the @dynamicCallable attribute, which enables nominal types to be "callable" via a simple syntactic sugar. This is the implementation of SE-0216. Read the proposal for ...