From what I've heard (from podcasts) the challenge of C++ Builder is catching up with Clang to add their additional C++ language features.

That doesn't seem fair. Apple had the same issues with their native clang compiler for a decade; and they were the primary sponsor/host of it.

The open source version will always be bleeding edge compared to any commercially packaged version. C++17 support, in general, is pretty good for a visual editor. IMO, at least.

They should open source and upstream their patches to clang and llvm, and let the llvm community maintain it.

That doesn't pay the bills. Cases of profitable open source projects are very scarce and difficult to achieve.

I was referring to the changes they made to the compiler in their fork of clang, which makes the "catching up" with upstream clang challenging as they need to rebase.

They can still sell their IDE and runtime libraries.

What they're adding are non standard C++ extensions only used by C++ Builder (properties, etc). There might be little benefit (and just maintenance cost) to upstream those to Clang. Clang also moves fast while it appears C++ Builder is still at C++17 [0].

[0] https://en.cppreference.com/w/cpp/compiler_support/20