when compiling ROOT with CMake with:
I can see how all compile commands contain:
just as expected. However, after finishing, I get:
So the standard shown here is a different one.
According to https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-108 that is indeed the same for gcc (but c++1y is deprecated and hence may be dropped in the future). It may not be the same for all compilers, though, so if other packages linking against ROOT pick this up, they may break.
It seems this is deliberately rewritten in root-config.in.