addition assignment behaves surprisingly with union types

[rkirov]

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

Code

let x: string | number = 0;
x += ''; // ERROR(2,1): Type 'string' is not assignable to type 'number'.

Expected behavior:
Since this is ok

let x: string | number = 0;
x = x + ''; 

similarly (showing that this has to do with type-narrowing)

function f(x: string | number) {
  x += '';
}

and finally TS allows string and number additions, thus I expect the original to also be accepted by the compiler.

Generally treating x = x + 1 and x += 1 differently from type checking perspective is surprising, but maybe this is byproduct of other designed behaviors.

Actual behavior:
Error - Type 'string' is not assignable to type 'number'.

[RyanCavanaugh]

I'm not really sure if this is a bug per se or not, but it doesn't seem like something that needs time spent on it either way.