Simple non-nullability detection for closure variables

[yahiko00]

Code

function foo() {
    let icon = document.getElementById("icon");
    if (!icon) return;
    // From here, the local variable icon is defined and not null
    icon.addEventListener("click", () => {
        let bar = icon.id; // error TS2531: icon is possibly null
    }, true);
}

Expected behavior:
In the code above, the local variable icon cannot be null after the if statement, unless I miss something. Although there is a closure in the addEventListener call, and this is an asynchronous function, I fail to see how and when such a variable icon could be null since there is no other reference to this variable until the end of the function foo. I am wondering how difficult it would be to check the non-nullability of closure variables in such a simple case. This would prevent some useless errors/warnings and would avoid the usage of the ! operator.

Actual behavior:
The compiler emits an error TS2531: Object [icon] is possibly null

[mhegazy]

declare icon as const instead. see #21162.

[yahiko00]

Alright, for my use case, this workaround is enough. Although, I suspect one could need to declare some variables (let) in some specific cases.

[ghost]

How about if icon is an argument of the function, e.g. foo(icon: HTMLElement): void ?