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?

Inizia riproducendo l'esempio TypeScript fornito nel Playground collegato e confrontalo con le issue correlate #12113, #33319 e #9998. L'issue non indica alcun file sorgente né test; il lavoro sarà concluso quando si sarà raggiunto un accordo sul fatto che questo narrowing basato su const debba rimanere valido all'interno della closure e il comportamento concordato sarà dimostrato con un esempio che compila.