Lost some time debugging a build issue which I was unable to reproduce. Turns out I was on an older version of clang despite both of us running the same version of macOS Catalina. Though you install the command line tools using xcode-select --install, there’s no way to force a reinstall with the tool as rerunning the command will tell you xcode-select: error: command line tools are already installed, use "Software Update" to install updates.
So updates are managed via the Software Update section in System Preferences and macOS reckons I’m up to date.
You can remove /Library/Developer/CommandLineTools and rerun xcode-select --install at which point you’ll obtain the latest version of command line tools. As a bonus while the install is in progress, macOS will serve a notice that an update is available and pop up the Software Update section in System Preferences.
When the initial install process invoked by running xcode-select complete, the update offered via Software Update disappears and it goes back to reporting everything is up to date.
While these two events were happening I wondered why the initial download was clocking up hours to download a 451MB file so I fired up tcpdump to see if there was any traffic coming through, turns out actually my machine was very busy downloading from an IP address of my ISP via plain HTTP. I initially wrote this post thinking that command line tools are not updated across major OS version upgrades, but I’m now wondering if the cache at the ISP is stale which is why I do not have the update. I also was not served the iOS 14.3 update notice until sometime last week though it was relased over a month ago!