The language reference makes use of the phrase "safety-checked undefined behavior" to mean "that which is undefined behavior in ReleaseFast and ReleaseSmall modes but will panic in Debug and ReleaseSafe modes".

This is a bit problematic because in the safe build modes, it's actually completely well-defined behavior. It will call the panic handler. Further, it will cause confusion because many people see "undefined behavior" and think it's a weakness of the language, when really it's the opposite - e.g. catching integer overflow bugs wouldn't be possible if it were always defined to be two's complement wraparound arithmetic.

I think a better phrase would be "illegal behavior". Illegal Behavior is always Undefined Behavior in the unsafe build modes. Illegal behavior at compile time is always a compile error. In safe build modes, runtime safety checks attempt to detect illegal behavior, but not all kinds of illegal behavior can be detected. Detected illegal behavior is well-defined and calls the panic handler. Undetected illegal behavior is undefined behavior, even in the safe build modes.

Instead of "safety-checked undefined behavior" the docs would say "detectable illegal behavior".

Related: #1966 #2301