It's not the documentation change that's the problem. It's the fact that they changed implementation-defined behavior to undefined behavior in the first place (then changed the documentation to follow). Or equivalently, they changed "recommended" to "required".
In particular, the "without ill effects" is no longer true. It's possible to #ifdef to detect broken libc/compiler combinations, but I'm not confident that avoiding explicit use of __builtin_dynamic_object_size will prevent optimizations from taking advantage of false assumptions based on __attribute__((malloc)).
In particular, the "without ill effects" is no longer true. It's possible to #ifdef to detect broken libc/compiler combinations, but I'm not confident that avoiding explicit use of __builtin_dynamic_object_size will prevent optimizations from taking advantage of false assumptions based on __attribute__((malloc)).