[fjfaase]

Is comparing a signed integer with an unsigned integer UB? I resently wrote some code and compiled it with gcc to x86_64 (without optimization) that returned an incorrect answer.

[Karliss]

No UB, but the integer promotions rules apply.

When comparing signed and unsigned integers of same size the signed one will be converted to unsigned. In a reasonably configured project compiler will warn about it.

In case of integers smaller than int, promotion to int happens first.

In case of signed and unsigned integers of different size, the smaller one will be converted to bigger one.

[benchloftbrunch]

It's not UB. Integer promotion applies, the signed int is implicitly coerced to unsigned (or the other way around - don't remember which.)