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

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

Duplicate of #9998.

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).

What's the conclusion of this?

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

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

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Narrowing down the type of a parameter isn't recognized in closure #34669

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Narrowing down the type of a parameter isn't recognized in closure #34669

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MartinJohns commented on Oct 23, 2019

fatcerberus commented on Oct 24, 2019

Buntelrus commented on Nov 20, 2020

MartinJohns commented on Nov 20, 2020

Buntelrus commented on Nov 20, 2020

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