>That's why array indexing is signed in C

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

C11: https://port70.net/~nsz/c/c11/n1570.html#6.5.2.1