[nwallin]

> Updating a large C++ program to newest version of Visual C++, for example, can be a mess.

That's an MSVC problem. MSVC ignored the C++ specification for decades. Now it does follow the spec. So a lot of non-standard code broke.

The transition was pretty ugly, as I'm pretty sure you've figured out. The permissive mode never gave warnings for non-standard code, so you couldn't just fix warnings as they came up. You had to do a fix the world update, which are a dickpain in large codebases. The transition was relatively fast; VS2017 introduced the standard compliant parser, and C++20 requires it.

Honestly MSVC is such a mess. I have a hunch that before this decade is out, Microsoft will just replace it with Clang a la Internet Explorer/Edge/Chromium. That's why the switch to the standard compliant parser was so rushed; they're trying to force everyone to write standard compliant code so that Clang can compile it.