Huh. According to https://en.cppreference.com/w/cpp/container/vector/operator_..., “no bounds checking is performed.” I find cppreference.com generally trustworthy, but maybe it’s wrong here? Or, maybe “no bounds checking” actually means “no guaranteed bounds checking?”
It means “no guaranteed bounds checking”, the standard only requires at() to throw if out of bounds (§ 26.2.4.1, note 15), but leaves unspecified how operator[]() should behave in invalid accesses, only that it isn't allowed to throw.
You need to look at the text of the Standard itself when it comes to this level of language lawyering. The short answer is that operator[] has undefined behavior if the index is out of range. Performing a check and terminating the program with some kind of runtime error is a legal subset of "undefined behavior", and so it's commonly done in debug builds, but you cannot rely on it in any sense.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n471...
Here is the Visual C++ documentation for bounds checking in debug builds.
https://docs.microsoft.com/en-us/cpp/standard-library/checke...