June 26, 2025 nullprogram.com/blog/2025/06/26/ C23 has a new rule for struct, union, and enum compatibility finally appearing in compilers starting with GCC 15, released this past April, and Clang later this year. The same struct defined in different translation units (TU) has always been compatible — essential to how they work. Until this rule change, each such definition within a TU was a distinct, incompatible type. The new rule says that, ackshually, they are compatible! This unlocks some ty...