Allow nested assignments in type guards

[ahejlsberg]

Fixes #8851.

[DanielRosenwasser]

👍 (if you don't care about compound assignment)

[DanielRosenwasser]

Technically you should consider the compound assignment operators here as well (e.g. +=, -= etc)

[ahejlsberg]

Compound operators aren't included in control flow analysis in our current design, so that would be a separate issue. But as I see it they wouldn't affect the type because they both a read and a write operation (you're basically storing a value of the same type back into the variable). Did you have something in mind?

[DanielRosenwasser]

Nothing other than odd cases like this:

var x: number | string = getNumberOrString();
x += "hello";
x; // currently has type 'string | number'

whereas in this case you get a different result

var x: number | string = getNumberOrString();
x = x + "hello";
x; // currently has type 'string'

I don't know how often something like that comes up in practice though.

[mhegazy]

👍