Taylor Blau [off-list ref] writes:

I am not saying that we should continue to write "if (0 <= k)", since we
will clearly never take the else branch. I am trying to say that
path_idx() *could* return (size_t)-1, and callers would be able to write
"if (k == (size_t)-1)" to check for that error condition.

My observation was that there are unlikely to be so many active paths at
any one time such that we'd ever want to return (size_t)-1 as a valid
index, and could always use it as an error sentinel.

Yes, I agree. If the list grows to (size_t)-1 entries, I think we also
run into other issues.

Anyhow, in the v3 I've submitted, I resolved it differently so no
awkward type cast is needed.

-- 
Cheers,
Toon