function assert(x) {
    if (!x)
        throw Error("Bad");
}

let isConstructor = createBuiltin("(function(c) { return @isConstructor(c); })");

// Functions.
assert(isConstructor(assert));
assert(isConstructor(class{}));
assert(isConstructor(function(){}));

// Proxy functions.
assert(isConstructor(new Proxy(assert, {})));
assert(isConstructor(new Proxy(class{}, {})));
assert(isConstructor(new Proxy(function(){}, {})));

// Bound functions.
assert(isConstructor(assert.bind(null), {}));
assert(isConstructor((class{}).bind(null), {}));
assert(isConstructor(function(){}.bind(null), {}));

// Builtin constructors.
assert(isConstructor(Array));
assert(isConstructor(ArrayBuffer));
assert(isConstructor(Boolean));
assert(isConstructor(Date));
assert(isConstructor(Error));
assert(isConstructor(Function));
assert(isConstructor(Map));
assert(isConstructor(Number));
assert(isConstructor(Object));
assert(isConstructor(Promise));
assert(isConstructor(Proxy));
assert(isConstructor(RegExp));
assert(isConstructor(Set));
assert(isConstructor(String));
assert(isConstructor(WeakMap));
assert(isConstructor(WeakSet));

// Non-function values.
assert(!isConstructor(undefined));
assert(!isConstructor(null));
assert(!isConstructor(true));
assert(!isConstructor(false));
assert(!isConstructor(0));
assert(!isConstructor(1));
assert(!isConstructor(1.1));
assert(!isConstructor(-1));
assert(!isConstructor(Date.now()));
assert(!isConstructor(new Date));
assert(!isConstructor(Infinity));
assert(!isConstructor(NaN));
assert(!isConstructor(""));
assert(!isConstructor("test"));
assert(!isConstructor([]));
assert(!isConstructor({}));
assert(!isConstructor(/regex/));
assert(!isConstructor(Math));
assert(!isConstructor(JSON));
assert(!isConstructor(Symbol()));
assert(!isConstructor(new Error));
assert(!isConstructor(new Proxy({}, {})));
assert(!isConstructor(Array.prototype));

// Symbol is not a constructor.
assert(!isConstructor(Symbol));

// Getters / setters are not constructors.
assert(!isConstructor(Object.getOwnPropertyDescriptor({get f(){}}, "f").get));
assert(!isConstructor(Object.getOwnPropertyDescriptor({set f(x){}}, "f").set));

// Arrow functions are not constructors.
assert(!isConstructor(()=>{}));

// Generators are not constructors.
assert(!isConstructor(function*(){}));

// Native builtins are not constructors.
assert(!isConstructor(isConstructor));

// https://tc39.github.io/ecma262/#sec-built-in-function-objects
// Built-in function objects that are not identified as constructors do not
// implement the [[Construct]] internal method unless otherwise specified in
// the description of a particular function.
assert(!isConstructor(Array.of));
assert(!isConstructor(Object.getOwnPropertyDescriptor));
assert(!isConstructor(Date.now));
assert(!isConstructor(Math.cos));
assert(!isConstructor(JSON.stringify));
assert(!isConstructor(Promise.all));
assert(!isConstructor(Symbol.for));
assert(!isConstructor(Array.prototype.push));

// Proxy and bound functions carry forward non-constructor-ness.
assert(!isConstructor(new Proxy(Symbol, {})));
assert(!isConstructor(new Proxy(Math.cos, {})));
assert(!isConstructor(Symbol.bind(null)));
assert(!isConstructor(Math.cos.bind(null)));