Support assignment in typeguard if block

[fdecampredon]

it would be nice to support that kind of construction :

function myFunction(arr: string | string[]) {
 if (typeof arr === 'string') {
 arr = [arr]
 }
 //arr is now string[]
}

It's a common pattern used by library that take options in parameter.
Generally you can pass an array of string or a string if you have a single value, and internally the library will create an array in any case.

[Deathspike]

I encountered the same thing. This would be a great addition to avoid unnecessary rewrites when adopting TS.

[falsandtru]

+1

[ivogabe]

I've opened a PR at #6959.

[RyanCavanaugh]

Works now