Object is possibly null inside a closure defined within a const constraint

Search: Object is possibly null, narrowed type, type constraint, closure

Code

const foo: string | null = 'bar'
if (foo) {
    function baz() {
        console.log(foo.length)  // Object is possibly 'null'
    }
}

Expected behavior:

Compiles

Actual behavior:

const foo: string | null
Object is possibly 'null'

Playground Link:

Playground

Related Issues:

#12113, #33319, #9998

Discussion:

By design, type narrowings are reset inside closures. This makes sense, but there is one case where better assumptions can be made: when the closure is defined inside a type constraint block AND the type narrowing is based on a const value. Even if called from async, the closure is still referencing a const and was created after a suitable type constraint occurred.

Thoughts?

Start by reproducing the provided TypeScript example in the linked Playground and compare it with related issues #12113, #33319, and #9998. The issue names no source file or test; done would mean reaching agreement on whether this const-based narrowing should remain valid inside the closure and demonstrating the agreed behavior with a compiling example.