C23 updated the definition of the [] operator to disallow negative subscripts with array type. I think you have to explicitly convert the array to a pointer type now.
int a[2];
a[-1]; // not ok
(&a[0])[-1] // ok
best off having a bespoke type that understands how big the array its indexing is
The source of confusion is that unsigned is a terrible name. Unsigned does not mean non-negative. Its 100% complete valid to assign a negative value to an unsigned, it just fails silently.
If you want non-negative integers, then you should make a wrapper class that enforces non-negativity at compile and runtime.
C’s implicit casts are tripping you up. Unsigned ints can’t be negative, but C will happily let you assign a negative signed int to an unsigned int variable, but the moment it is assigned it ceases to be negative. In serious programming languages this implicit assignment is forbidden—you have to explicitly cast.
> For example, looping to the 2nd to last item in an array or getting the index before the given index.
I don’t understand what you mean here, can you clarify?
> If you want non-negative integers, then you should make a wrapper class that enforces non-negativity at compile and runtime.
Unsigned integers are the compile time side of the coin, but yes you may want to take care to enforce it at runtime as well, though this typically implies a performance penalty that most don’t want to pay.
You never want any element of an array, except elements within the range [0, array_length). Anything outside of that is undefined behavior.
I think people tend to overthink this. A function which takes an index argument, should simply return a result when the index is within the valid range, and error if it's outside of it (regardless of whether it's outside by being too low or too high). It doesn't particularly matter that the integer is signed.
If you aren't storing 2^64 elements in your array (which you probably aren't - most systems don't even support addressing that much memory) then the only thing unsigned gets you is a bunch of footguns (like those described in the OP article).
You can deal with this by casting before doing the subtraction, or you can deal with it by storing the indices as signed integers at all times. The latter is more ergonomic at the cost of wasted capacity.