just do everything in Isabelle and prove correctness, ezpz no tests required

Actually when it comes to C++ 23 library features, MSVC is ahead of both. In fact, as far as I can tell, MSVC is the only compiler that fully supports all C++ 20 core language features at the moment. So credit where credit is due, MSVC has gotten way way better the past few years. Visual Studio is still awful, but the compiler has become quite competent.