Property refinements are unsound

[vkurchatkin]

function test(x: { a: string | number }): { a: string } | undefined {
    if (typeof x.a === 'string') {
        return x;
    }
}

const a:  { a: string | number } = { a: 'foo' };
const b = test(a);

a.a = 1;

if (b !== undefined) {
    b.a.toUpperCase(); // runtime error
}

[JSMonk]

Yes. It's a big problem. We will try to fix it soon. Thank you for your contribution ^_^.