I'd accept that, too. I'm not familiar w/ Swift, so it wasn't in the list above. (But I do think the default should not be code units, or programmers will use it incorrectly out of ignorance. Forcing them to choose prevents that, hence, I'll allow it)
I would go one step further and say that there is no meaningful default for these things. It all depends on the context, and there's no single context that is so common that it's the only one that most people ever see. Thus, it should always be explicit, and an attempt to enumerate or index a string directly should not be allowed - you should always have to spell out if it's the underlying encoding units, or code points, or text elements, or something else.
