When I use a declaration that's marked @available(…) as of a newer SDK than the deployment target, I get a diagnostic like this:

    app_controller_mac.mm:414:14: error: 'setAllowsAutomaticWindowTabbing:' is only available on macOS 10_12 or newer [-Werror,-Wunguarded-availability]
    [snip]
    app_controller_mac.mm:414:14: note: enclose 'setAllowsAutomaticWindowTabbing:' in an @available check to silence this warning
        NSWindow.allowsAutomaticWindowTabbing = NO;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

But if I try to use @available, I get another diagnostic that suggests it isn't implemented:

    ../../chrome/browser/app_controller_mac.mm:412:7: error: cannot compile this scalar expression yet
      if (@available(macos 10.12, *))
          ^~~~~~~~~~~~~~~~~~~~~~~~~~

It looks like this first phase of @available was added here:
https://reviews.llvm.org/D23003?id=67713