Name collisions in intersection types should be of a union type

[attatrol]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)

Code

const x = Object.assign({}, {type: '42'}, { type: 42 });
let type = x.type;
type.substring(0); // runtime failure

Expected behavior:
Variable type should be number | string.
Actual behavior:
It is number & string.

In general we cannot presume that every function that produces intersections will also intersect any properties.

[mhegazy]

The use of intersection in Object.assign is an approximation. the correct behavior is that of spread, i.e. the type for the property in the last argument overrides those of the previous argument. that is tracked by #11100.
We first need higher-order spread operator to be added to the language, which is tracked by #10727

[mhegazy]

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