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@sqx @vampiress At least GCC does warn:
<source>: In function 'f':
<source>:3:21: warning: operation on 'stack_height' may be undefined [-Wsequence-point]
3 | stack[stack_height++] = stack[stack_height-1];
| ~~~~~~~~~~~~^~

clang warns too.
I almost always use `-W -Wall` (-W is an alias for -Wextra) when compiling which is why I see these warnings.
Also there is a paper for C to make it well defined and not unspecified.