I think such benefits can be used in some CI testing
Yes, I agreed. Most of the features related to recent GCC version automatic checks may be implemented in CI process. Yes, definitely we can improve automatic checks in CI builds.
There can be cases when one wants to replace a RPM installation with a git-branch
As options, we can check GCC version used on RPM dist during compilation to make sure used not too old GCC. And print command that allows installing recent GCC.
For other coins, we can check GCC documentation. Maybe you will find something useful in the section "New Languages and Language specific improvements" [GCC9 release changes](https://gcc.gnu.org/gcc-9/changes.html) [GCC8 release changes](https://gcc.gnu.org/gcc-8/changes.html) [GCC7 release changes](https://gcc.gnu.org/gcc-7/changes.html) [GCC6 release changes](https://gcc.gnu.org/gcc-6/changes.html) [GCC5 release changes](https://gcc.gnu.org/gcc-5/changes.html)