RFC: Raise the minimum compiler requirements to move toward C++20

[h-vetinari]

I had bookmarked the following summary from the previous thread:

@jyknight

Until Clang 17, support for C++20 was extremely buggy. I would not recommend that anyone use previous versions in C++20 mode – it’s just not worth the problems.

There were multiple serious issues for consteval, concepts, and other new features. For Google’s internal codebase, these caused problems just trying to switch from -std=c++17 to -std=c++20 – including silent miscompiles. Our codebase didn’t even use C++20 features, we were just switching the compilation mode (which introduced usage of C++20 features via the stdlib headers.)

For reasonable C++20 compiler support, I’ve been making a recommendation of the following minimum compiler versions:

• Clang 17
• GCC 12
• MSVC 17.2

Clang 17.0.0 was released in Sept. 2023, so not yet close to 3 years – OTOH, the developer policy also notes “we may […] decide to support fewer versions.”

C++20 is somewhat special because of how big of an increment it was. A good illustration of this is the massive gap between when support was ready for C++17 resp. C++20; from clang’s C++ conformance page

LLVM 5.0.0 was released in September of 2017, so it took 6 years to get even baseline support in. Given the timelines there, the project IMO could decide to just jump to a minimum clang 17 support.

FWIW, I’ve never been quite convinced by the argument about the existence/size of the demography that is supposed to be protected by keeping things compilable on old distros. If someone wants to build LLVM from source (a pretty advanced task in itself), surely – for the most part – they are also able to get a newer compiler from their distro (or some other channel; there are various), or build one locally.