With this PR a parameter is implicitly considered const when there are no assignments to the parameter anywhere in the declaring function (or functions nested within it). Similar to const variables, when a parameter is considered const, type guards for the parameter remain in effect in nested function expressions and arrow functions:

function f1(x: Object) {
    if (typeof x === "string") {
        setTimeout(() => console.log(x.charAt(0)), 0);
    }
}

function f2(info?: { name: string }) {
    if (!info) {
        return;
    }
    setTimeout(() => console.log(info.name), 0);
}

Above, x and info are implicitly const because there are no assignments to them anywhere in their containing functions, and therefore the type guards remain in effect in the callback functions.

Fixes #10180. Also see #7719 and a number of similar issues.