Optional field type narrowing should make types compatible

[rasmushalland]

TypeScript Version: 2.5.2

Code

type mytype = {
    p1: string;
}

function constructMyType_error(): mytype {
    let o: Partial<mytype> = {};
    o.p1 = 'x';

    // Error "Type 'undefined' is not assignable to type 'string'"
    return o;
}

function constructMyType_noerror(): mytype {
    let o: Partial<mytype> = {};
    o.p1 = 'x';

    // No error.
    return { p1: o.p1 };
}

Expected behavior:
No compiler errors.

Actual behavior:
A compiler error in the function constructMyType_error, although the compiler knows that the field p1 has type string instead of string | undefined. It does not use this knowledge to conclude that the variable o is compatible with the return type.

Why is this problematic
In some cases it is nice to construct an object one property at a time. Without loosing any compiler checks, that is. And without having to write the code to copy the object, as the function constructMyType_noerror does.

[RyanCavanaugh]

See #10065

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.