[JdeBP]

... and indeed on several of the BSDs.

It has been the default compiler in base on FreeBSD since version 10, and FreeBSD is now built with it. There was also a push back around 2014 or so to get much of the ports tree built with it, too.

DragonFly BSD has been buildable with clang since around 2014, albeit using clang from packages/ports. In 2017 (release 50) the DragonFly people started work on pulling it into base.

OpenBSD switched building on x86/amd64 to clang in 2017 and building on armv7 to clang in 2018.

NetBSD has included clang in base but as far as I know does not (yet) build with it by default on any architecture.

[simias]

I only mentioned Linux because I knew that my argument wouldn't work for BSDs! But in my defense the situation, at least originally, was more political than technical (at least as far as FreeBSD was concerned, I don't follow the other BSDs too closely). GCC switching to the GPLv3 was a big no-no for many people in the BSD world. As a result they were at the forefront to port the system to clang since they were stuck with an obsolete GPLv2 GCC version.