Don't we always generate a warning/error on this case, even with None? If not, I think we can fix it instead of documenting it, by changing the platform.txt options...

only 3 out of 4 options produce error:

'None' never does, -w overrides any -W flags since they apply all at once and not like 'disable all warnings but still enable this one', order does not matter. Removing -w from 'None' kind of defeats the purpose of the option, and there is 'Default' for that.

We do still need to mention the warnings, which even in 'Default' will give some useful advice. Specifically with C++, shouldn't we mention that it is undefined behaviour when trying to code like that?

btw low-level routines that really do not return may still be coded like this

int func() {
  __builtin_unreachable();
}

Argh, you are correct. Even trying to explicitly enable a warning like -w -Wreturn-type -Werror=return-type doesn't work.

It seems the only way to make this automatically catch is to not use -w and specifically list all ~100 -Wno-XXX in an @option file. While doable, that's more involved than I have time for this AM...