How does readline use compgen to get all applicable completions?

I'm building an alternative to readline and would like to have tab completion in a similar way to how readline has tab completion. In readline, if you type git you get all the git subcommands as autocomplete options. But if you type ls you get files and directories. It's pretty clear how to get those individually by invoking compgen using one of it's options, but it's not clear how to invoke compgen and get the same set of completions that readline gives. Essentially completeing from all applicable sources. What option can I supply to get that behaviour?