[SAI_Peregrinus]

C implicitly converts integer types as needed. The specific conversion rules are rather convoluted, see section 6.3 of the ISO C standard for the details. Before C23 this was implementation defined, but since C23 now mandates two's complement representation of signed integers and wrapping for unsigned overflow (as before) the `-1` is converted to UINT_MAX. Then the `%d` format specifier performs a conversion from `unsigned int` back to `int` for printing, resulting in `-1`. Implicit conversions between types of the same rank are guaranteed to "round-trip", you get back the value you started with when converting back to the starting type. If they'd specified an unsigned format specifier for printing then they wouldn't get `-1` printed.

[gpderetta]

I'm pretty sure that this was well defined and portable well before C23. Although the representation was implementation defined, IIRC signed to unsigned has always been guaranteed to be as-if in two-complement.

[SAI_Peregrinus]

You're probably correct, but I didn't have the earlier standards as quickly to hand to cite. The storage representation being twos complement is new, I didn't mean to imply that the conversion was as well.