Type narrowing is not allowed if variable is reassigned after the use site inside if block

[vladima]

Current implementation is very conservative when looking for assignments during application of type narrowing.

function len(x: number | string) {
     if (typeof x === 'number') {
         var t = x.toExponential();
         //x = "aaaa"; // uncomment me
     }
}

variable is reassigned after the usage of narrowed type but type narrowing still not kicked in

[ahejlsberg]

We'd have to do flow analysis to properly reason about this. That might be worthwhile (and would allow us to detect unreachable code, unassigned variables, etc. as well), but we need to consider the impact on complexity and performance.

[vladima]

btw, there is already a PR for control flow analysis: #1287

[jbondc]

Control flow looks really nice 👍

[mhegazy]

So what is the proposal?

[ivogabe]

I've opened a PR #6959 that will fix this.

[ivogabe]

I think that this can be closed, #8010