Compound assignments and narrowing

[zhuravlikjb]

Noticed a very strange behaviour with compound assignments and narrowing.

This works fine with a normal assignment:

function moooo12() {
    var x: string | number = 5;
    x = x + "a";  // RHS "x" is narrowed, LHS "x" is not, everything right
    x;
}

This doesn't work, though:

function moooo12() {
    var x: string | number = 5;
    x += "a"; // the compiler thinks that "x" is "number" here, and states that "string" is not assignable to it
    x;
}

Seems that in the last case the compiler shouldn't narrow the LHS "x" anyways, but rather it should specially calculate the result type of "+=" assignment taking into account the narrowed version of "x".

[DanielRosenwasser]

@ahejlsberg and I have actually discussed this: #9013 (comment)

Is there a use case you had in mind @zhuravlikjb?

[zhuravlikjb]

@DanielRosenwasser No particular use cases. I was supporting some corner cases in my own type guards mechanism, and stumbled upon the fact that the TypeScript compiler behaves strangely with compound assignments. I'm always a source of theoretical questions, sorry. :)

[DanielRosenwasser]

No worries! Looks like we have that in common. 😄

[RyanCavanaugh]

Closing due to lack of additional interest. The behavior here seems at least sometimes desirable.