would this help with your problem with modules, it's referenced on the book's page:
https://www.stroustrup.com/module_use.html ?
> CGG (sic) uses .cxx for module files Use -fmodules to use modules.
no it does not. Trying this on Fedora 39 for example, my original comment above already had -fmodules-ts, and when I use -fmodules, g++ errors with
g++: error: unrecognized command-line option ‘-fmodule’; did you mean ‘-Mmodules’?