Problem:
By default, PlatformIO uses -std=gnu++11 as a compiler flag but you want to use C++17 or C++23 features.
If you just use
build_flags = -std=gnu++17
this will lead to g++ being called with g++ ... -std=gnu++17 ... -std=gnu++11 ... compiler flags. The latter one – gnu++11 i.e. C++11 will take precedence.
Solution:
In order to activate C++17, use
build_flags = -std=gnu++17 build_unflags = -std=gnu++11
In order to activate C++23 (not fully implemented yet in G++), you currently need to use -std=gnu++2a:
build_flags = -std=gnu++2a build_unflags = -std=gnu++11