Pick<T, xxx> sometimes considered an empty type

[ahejlsberg]

TypeScript Version: 3.3.0-dev

Code

function test<T extends { a: string }>(obj: T) {
    let { a, ...rest } = obj;
    return { ...rest, b: a };
}

let o1 = { a: 'hello', x: 42 };
let o2: { b: string, x: number } = test(o1);  // Error, but shouldn't be

Expected behavior:

No error.

Actual behavior:

Error as shown above.

Playground Link:

https://www.typescriptlang.org/play/index.html#src=function%20test%3CT%20extends%20%7B%20a%3A%20string%20%7D%3E(obj%3A%20T)%20%7B%0D%0A%20%20%20%20let%20%7B%20a%2C%20...rest%20%7D%20%3D%20obj%3B%0D%0A%20%20%20%20return%20%7B%20...rest%2C%20b%3A%20a%20%7D%3B%0D%0A%7D%0D%0A%0D%0Alet%20o1%20%3D%20%7B%20a%3A%20'hello'%2C%20x%3A%2042%20%7D%3B%0D%0Alet%20o2%3A%20%7B%20b%3A%20string%2C%20x%3A%20number%20%7D%20%3D%20test(o1)%3B%0D%0A

Related Issues:

Found while investigating #29062.