JSTests/stress/proxy-instanceof.js

function assert(b) {
    if (!b)
        throw new Error("Bad assertion");
}

function test(f) {
    for (let i = 0; i < 1000; i++)
        f();
}

let constructors = [Error, String, RegExp, function() {}, class C {}];

for (let constructor of constructors) {
    test(function() {
        let proxy = new Proxy(constructor, {});
        assert(new constructor instanceof proxy);
    });
}

test(function() {
    let called = false;
    let proxy = new Proxy(function(){ called = true; }, {});
    assert(new proxy instanceof proxy);
    assert(called);
});

test(function() {
    let called = false;
    let handler = {
        get: function(target, prop) {
            if (prop === "prototype")
                return {};
            return target[prop];
        }
    };
    let proxy = new Proxy(function(){ called = true; }, handler);
    assert(!(new proxy instanceof proxy));
    assert(called);
});