Narrowing down the type of a parameter isn't recognized in closure

[jmankopf]

TypeScript Version: v3.7-Beta

Search terms : narrow narrowing down type parameter closure

Code

function foo(arg: string | undefined) {
    arg = arg ?? "default";

    arg.length // no error
    const bar = () => {
        arg.length; // error 
    }
}

Expected behavior:
arg should be of type string within the body of the closure.

Actual behavior:
arg is possibly 'undefined' within the body of the closure.

Playground Link: Playground link

[MartinJohns]

Duplicate of #9998.

[fatcerberus]

Is this actually a duplicate of #9998? That's a design limitation wherein narrowings are optimistically not reset when calling a function because it would be impractical to follow the control flow into the call. Here, the narrowing is being reset at a closure boundary, which is normal (i.e. not just a design limitation - it's deliberate).

[Buntelrus]

What's the conclusion of this?

[MartinJohns]

The conclusion is that it's a duplicate of #9998.

[Buntelrus]

I don't get it. What can I do to solve this problem?