Union doesn't merge objects properties' type

[nicolo-ribaudo]

TypeScript Version:

This is reproducible both in VSCode 1.0 and in the playground (https://www.typescriptlang.org/play/#src=...)

Code

interface A {
    x: number;
}

interface B {
    x: string;
}

const c: A | B = { x: 1 || "a" };

const d: A | B = { x: 1 } || { x: "a" };

Expected behavior:
Both c and d should be fine

Actual behavior:
TypeScript reports this error for the const c: A | B = { x: 1 || "a" }; statement:

Type '{ x: number | string; }' is not assignable to type 'A | B'.
  Type '{ x: number | string; }' is not assignable to type 'B'.
    Types of property 'x' are incompatible.
      Type 'number | string' is not assignable to type 'string'.
        Type 'number' is not assignable to type 'string'.

[mhegazy]

{ x: number | string }; is neither A nor B. it becomes easier to see that with more than one property, e.g.

interface A {
    x: number;
    y:  number;
}

interface B {
    x: string;
    y: string;
}

const c: A | B = { x: 1 || "a" , y :  2 || "b"};

[RyanCavanaugh]

I should add that we've gotten this request around single-property types a bunch. It turns out to be very difficult to detect all of these cases properly in the compiler.

[nicolo-ribaudo]

@mhegazy Thank you, now I see why the current behavior is correct