GCC was designed to be extensible and portable (after all it ended up ported to more architectures than any other compiler). The political limitation was more subtle.

Originally FSF did not want to make it easy let GCC store its intermediate language and read it back. That was because it would let others to build proprietary frontends and back-ends which is against RMS' vision. This has changed and LTO does precisely that (and indeed in addition to technical difficulties this political issue delayed its arrival in GCC). So this political block of extensibility is long gone (and I am happy for that).

A lot has changed in compilation between mid 1980's GCC was started and early 2000's. LLVM design reflects its time. Almost 20 years passed since that and both projects needs to evolve and develop strategies of doing so.

Regardless of the LTO relaxation, I believe it is still the position of the FSF and GCC that any middleware that links in to GCC itself is GPL-infected. Obviously, this is not the case for LLVM/Clang.

GCC was intentionally designed to be hard to extend with proprietary extensions (there's an email from RMS somewhere explicitly stating that)

Whereas llvm/clang was designed as a library to be open and extensible, which lead to a lot of tooling being built around it.

Unrelated to license, clang also has far better error messages than gcc.

> clang also has far better error messages than gcc.

This was true a number of years ago, but I believe GCC's diagnostics have gotten much better in response to competition and don't believe it to be true anymore (although my experience is with C code, not C++ — maybe Clang is better there).

Another distinction is that GCC continues to hold a significant lead in producing accurate debuginfo, which is invaluable to me as a developer.

I think both compilers give good error messages (C++14 and C++17). Sometimes one is clearer, sometimes the other, so what I usually do if I can't immediately figure out what the compiler is complaining about is to try the other one and see if that clears things up or at least narrows it down a bit. Works surprisingly often :)