Spread operator should declare members as undefined for null or undefined objects

[stephanedr]

TypeScript Version: 2.1.4 & 2.2.0-dev.20161220

Code

function test(t?: { a: number } | null): void {
    let b = { ...t };  // b: { a: number }
    let c: number = b.a;  // no error
}

Expected behavior:
b.a to be number | undefined, as if t is null or undefined, b is an empty object.
Actual behavior:
b.ais number.

This can also cause a value of a completely wrong type to be accepted!

interface I {
    "foo-bar": "foo" | "bar";
}

function f(x: I | undefined): I {
    return { "foo-bar": 123, ...x }
}

console.log(f(undefined));

[sandersn]

getSpreadType currently filters null and undefined before lifting unions. It should wait until after unions are lifted so that, for example, the type of the spread in @Andy-MS's example is { "foo-bar": number | string }. I should have a fix out soon.

[sandersn]

Fix is up at #16306