delete doesn't count as turning a property to undefined

function plus1(x) {
    return x + 1;
}
let y: { z: number } = {
    z: 5
};
delete y.z; // if you switch this line to y.z = undefined it will produce an error
let result = plus1(y.z);

This should return an error because type undefined is not compatible with type number

Also lovely work so far!

[Mathspy]

Sweet! Thank you for the amazing work! ❤️

[JSMonk]

Unfortunately, there is no kind of list. So, if you will report it in issues, it will be great.