This is not the case. They are discussing the default value of `g++ -std=...`. That does not complicate bootstrapping as long as the C++ sources of GCC are compatible with older and newer versions of the C++ standard.
I've worked on a number of pretty large projects. If the target for the source code changes it can be really hard to keep C++20 features from creeping in. It means that you either need to explicitly build targeting 11, or whoever does code reviews needs to have encyclopedic knowledge of whether or not a change leaked in a future feature.
It is "doable" but why would you do it when you can simply keep the compiler targeting 11 and let it do the code review for you.
It doesn't appear to me that the parent comment was implying otherwise.
The default is changing for any compilation that doesn't explicitly specify a standard version. I would have thought that the build process for a compiler is likely careful enough that it does explicitly specify a version.
I could be misreading this, but unless they have a different understanding of what it means to dog fooding than I do then it seems like the proposal is to use C++20 features in the compiler bootstraping.
The email mentions that the last time they changed it was 5 years ago in GCC 11, and the link <https://gcc.gnu.org/projects/cxx-status.html#cxx17> indeed says
> C++17 mode is the default since GCC 11; it can be explicitly selected with the -std=c++17 command-line flag, or -std=gnu++17 to enable GNU extensions as well.
which does not imply a change in an obscure feature (bootstrapping) that would only affect a few users.
Even if you only target 11, there may be advantages to setting a newer version anyway. Sometimes the standard finally allows some optimization that would work, or disallows something that was always error prone anyway. I would recommend you set your standard to the latest the compiler supports and fix any bugs. Solve your we have to support older standards problem by having your CI system build with an older compiler (and also the newest one). C++ is very good at compatibility so this will rarely be a problem.
For example in CMake the natural variable is CMAKE_CXX_STANDARD, but it's implemented backwards: if you set it to 14 but your compiler supports only C++11, they'll add -std=gnu++11. You have to also set CMAKE_CXX_STANDARD_REQUIRED to ON, which not man projects do. I don't think there's an easy way to say "this project requires C++14 or higher".