I assumed that the specified language features, especially ones also depending on core libraries like libc++, would most likely be released as part of the next major OS update (since these are very essential for the complete system and need to be tested along side it as one whole).
I didn't check the macOS 10.13.4 beta, but I'm assuming that the features I mentioned are coming as part of macOS 10.14 and Xcode 10.
I assumed that the specified language features, especially ones also depending on core libraries like libc++, would most likely be released as part of the next major OS update (since these are very essential for the complete system and need to be tested along side it as one whole).
I didn't check the macOS 10.13.4 beta, but I'm assuming that the features I mentioned are coming as part of macOS 10.14 and Xcode 10.