Missing excess property error with spread

[ghost]

TypeScript Version: 3.1.0-dev.201xxxxx

Code

function f(o: { a: number, b: number }): { a: number } {
    return { b: 0, ...o };
}

Expected behavior:

Error on b.

Actual behavior:

No error.

Broken by #26798.

Related Issues: #27166, #23340

[sandersn]

@weswigham points out that this is probably correct; b: 0 is not excess because it's overridden by o.b which comes from a spread, so should not count as excess.

What would be more useful is an error for unused properties in a spread: any property that is not used because of later spreads. The error should only apply to non-spread properties.

[WreckedArrow]

I'm not sure if this is related or has been addressed elsewhere, but this also does not produce an error:

function f(o: { a: number, b: number }): { a: number } {
    return o;
}

[ghost]

@WreckedArrow That's #12936