Comment by motorest

> One of the projects came back with 'just flipping C++20 switch with no changes causes significant regression on build times'.

I think this just proves that your team is highly inexperienced in C++ projects, which you implicitly attest by admitting this was your first C++ upgrade you had to go through.

Let me be very clear: there is never an upgrade of the C++ version targeted by a project that does not require full regression tests and a few bugs to squash. Why? Because even if the C++ side of things is perfectly fine, libraries often introduce all sorts of unexpected issues.

For example, once I had to migrate a legacy project to C++14 and flipping the compiler flag to c++14 caused a wall of compiler errors. It turned out the C++ was perfectly fine, but a single library behaved very poorly with a constexpr constructor they enabled conditionally with C++14.

You should understand that upgrades to the core language and standard libraries are exceptionally stable, and a clear focus of the standardization committee. But they only have a say in how the core language and standard libs should be. The bulk of the code any relatively complex project consumes is not core lang+ stdlib, but third-party libraries and frameworks. These often are riddled with flags to toggle whole components only in specific versions of the C++ language, mainly for backwards compatibility. Once you target a new version of C++, often that means you replace whole components of upstream dependencies. This often requires fixing your code. This happens very frequently, even with the likes of Boost.

So, what you're complaining about is not C++ but your inexperience in software engineering in general. I mean, what is the rule of thumb about major version upgrades?