docs: One of lifetime elision examples says "output lifetime is unclear" but not "ILLEGAL"

[dequis]

http://doc.rust-lang.org/book/lifetimes.html#examples

fn frob(s: &str, t: &str) -> &str; // ILLEGAL, two inputs
fn frob<'a, 'b>(s: &'a str, t: &'b str) -> &str; // Expanded: Output lifetime is unclear

All other examples say ILLEGAL clearly, and when i read it, it seemed like the expanded version was legal, even though the output lifetime is unclear, which sounded a lot like the kind of "undefined behavior" rust wants to avoid, and that made me feel very uncomfortable.

If this is actually illegal, it would be nice to say it explicitly.

[nagisa]

I’d actually replace “unclear” with “ambiguous”.