Object spread should not exclude setter-only properties

[ajafff]

TypeScript Version: 3.1.0-dev.20180808

Search Terms:

Code

const obj = {
    foo: 1,
    ...{set bar(v: number) {}}
};

Expected behavior:

obj is inferred as {foo: number, bar: undefined}.
Spreading a setter-only property uses undefined as the property's value.
Likewise the following evaluates to true:

({
    foo: 1,
    ...{set foo(v) {}}
}).foo === undefined;

Actual behavior:

obj is inferred as {foo: number}.

Playground Link: https://agentcooper.github.io/typescript-play/#code/MYewdgzgLgBCBGArGBeGBvAUDHMBmIIAXDAIwA02uAdLehAKazwCGATgBQBuJYArgFt4DNgEoMAXwmYJAbkxA

Related Issues:
#11596
#21759
#17793

[ajafff]

This might be unnecessary if #26338 lands. The special handling of setter-only properties could then be removed completely

[sandersn]

From the type-level perspective:

A set-only property on the rhs introduces a property of type undefined that does not union with the same property's type from the lhs.