function shouldBe(actual, expected)
{
    if (actual !== expected)
        throw new Error('bad value: ' + actual);
}

let map = new Map();
for (let i = 0; i < 5; ++i)
    map.set(i, i);

for (let i = 0; i < 1e2; ++i) {
    let cloned = new Map(map);
    shouldBe(cloned.size, map.size);
}

class DerivedMap extends Map {
    constructor(map)
    {
        super(map);
    }

    set(key, value)
    {
        // ignore.
    }
}

for (let i = 0; i < 1e2; ++i) {
    let cloned = new DerivedMap(map);
    shouldBe(cloned.size, 0);
}