Build instructions say:
https://github.com/SerenityOS/serenity/blob/master/Meta/Lagom/ReadMe.md?plain=1#L19-L26

to build with clang, but doesn't specify a version. The code is full of c++ 20 decorators [[likely]] or [[unlikely]]

A cmake failure should trigger if your version on clang is too old to support these similar to this one for versions of gcc older than 10.2:

https://github.com/SerenityOS/serenity/blob/master/Meta/Lagom/CMakeLists.txt#L11-L15

I found that Clang-10 failed to build Clang-12 succeeded. I did not test clang-11.