Making realloc(buff, 0) undefined means that the compiler is permitted to do nothing instead of calling free on buff. Because this both complies with the C23 standard and makes the computer do less work, I won't be surprised if we start seeing compilers applying this optimization. Hopefully we'll at least get a "-frealloc-size-zero-free" compiler flag or something.

BTW, if the "we" implies you're a WG14 member, I wanted to thank you for helping move the language forward. C23 has tons of nice features that makes it compelling to move to (#embed, true/false being keywords, binary literals, built-in endianness macros, etc), which is why I'm concerned about adding more undefined behavior to the standard because some BSD variants didn't comply with the previous standard.