int ctfeFun(const int[] arr) {
    auto startp = arr.ptr;
    auto endp = arr.ptr + arr.length;
    
    for(; startp < endp; startp++) {}
    
    return 0;
}

const arr = [1, 2, 3];
enum ans = ctfeFun(arr);

test9.d(5): Error: pointer expression startp++ cannot be interpreted at compile time
test9.d(11): Error: cannot evaluate ctfeFun(arr) at compile time

Changing startp++ to ++startp fixes the problem.