This is "Wrong" in the sense that C++ does really work like this, but it's not wrong in the sense that this is somehow unavoidably the case.

For example if you attempt an equivalent mistake in WUFFS that will be rejected.

Your WUFFS compiler will say this variable named slot must be a non-negative integer smaller than the length of the array, but as far as it can tell you didn't ensure that was true, therefore this code is nonsense, do better.

As I explained in my sister reply, in a broader context some of these are semantic properties and so there's a dilemma and C++ chooses to resolve that dilemma by accepting nonsense programs, but that wasn't the only available resolution and I am confident it's the wrong choice.