I'm pretty confused. I didn't know who Ted was, but a quick google search shows he is an OpenBSD dev and worked for Coverity. Coverity itself will flag this error. Now you are backing up that position. Historically, this exact thing has been the cause of many security vulnerabilities. It's especially precarious with the POSIX API due to many functions returning negative for error. I recall OpenSSH making sweeping changes to rid the code of signed ints being used for error, for this reason.
Can you explain why you would advocate this? Am I misunderstanding you, or missing something?
I replied to the other comment in this thread with an openbsd vulnerability caused by doing what is being advocated (I did choose openbsd to be funny).
Looks to me like the vulnerability you linked to demonstrates the exact opposite of what you think it demonstrates: The problem is that an unsigned value (a length) was being stored in a signed integer type, allowing it to pass a validation check by masquerading as a negative value.
Well, no, select() takes a signed value for the length (it is actually not a length, but the number of descriptors, later used to derive a length), and there is no changing that interface obviously. This is the source of the "negative value" in this example. The problem arises because internally, openbsd "jammed a negative value into an unsigned int", as Ted put it, and made it a very large positive value, leading to an overflow.