False Negative：ArrayIndexOutOfBounds.ql

Start with Likely Bugs/Collections/ArrayIndexOutOfBounds.ql and compare how it handles direct array access with accesses using helper methods, aliases, and computed indices. Reproduce the Java examples from the issue, then verify that the query reports each out-of-bounds access, including negative indices.

Version
codeql 2.23.9

When I detect the code like this using Likely Bugs/Collections/ArrayIndexOutOfBounds.ql, the problem is reported normally:

package scensct.core.pos;
public class PosCase1 {
    public static void main(String[] args) {
        int[] arr = new int[5];
        int index = 10; // Unbounded index, no constraint check before access
        int value = arr[index]; // Direct access with potentially out-of-bounds index // [REPORTED LINE]
    }
}

However, when I use a mediator variable or call a mediator function, ArrayIndexOutOfBounds.ql fails to detect the problem:

package scensct.var.pos;

public class PosCase1_Var4 {
    public static void main(String[] args) {
        int[] arr = createArray();
        int index = getIndex();
        int value = arr[index]; // Access with index from method
    }

    private static int[] createArray() {
        return new int[5];
    }

    private static int getIndex() {
        return 10;
    }
}

package scensct.var.pos;

public class PosCase2_Var4 {
    public static void main(String[] args) {
        int[] arr = new int[5];
        int K = 5;
        // Introduce an alias reference
        int[] alias = arr;
        int index = K;
        int value = alias[index];
    }
}

package scensct.var.pos;

public class PosCase2_Var5 {
    private static int getIndex(int k) {
        return k;
    }

    public static void main(String[] args) {
        int[] arr = new int[5];
        int K = 5;
        // Move index computation to a helper method
        int index = getIndex(K);
        int value = arr[index];
    }
}

package scensct.core.pos;

public class PosCase3 {
    public static void main(String[] args) {
        int[] arr = new int[5];
        int K = -1; // Negative bound
        int index = K + 0; // Index bounded below by negative K
        int value = arr[index]; // Access with potentially negative index
    }
}