They are already different language versions. They're specified in entirely different standards. I don't see what's left to be confused about. At most, perhaps the C++ standard committee could be criticized for repeatedly going out of their way to maximize backward compatibility.
> If every time they're going to add things, remove things and break things then we're in practice talking different strands.
They are already different standard. What's there to miss?
> I know you can do that at the linker and with makefiles and compile flags, this is about a more sane presentation.
This take doesn't make sense. The C++ version being used in a project is a property of the project, not of the translation unit or individual files. A project is comprised of multiple declarations and corresponding definitions, which are spread around and reused and make sense as a whole. It would not make sense to, say, have a translation unit comprised of X C++11 definitions mixed with Y C++20 definitions.
