Closure error TS2532

TypeScript Version: 2.6.2 | 2.7.0-dev.20180112

Code

// test.ts
function foo(map: Map<string, string>) {
    let bar = map.get('key');
    if (!bar) {
        return;
    }

    let res1 = bar.repeat(3);

    let fun = () => {
        let res2 = bar.repeat(3);
    }
}

Expected behavior:
no errors

Actual behavior:
test.ts(11,20): error TS2532: Object is possibly 'undefined'.

[ahejlsberg]

This is working as intended. Type guards remain in effect across function boundaries only when the guarded variable(s) are considered constant. This is the case for const local variables and parameters that never occur as assignment targets in the function, so if you declare bar using const it works as you expect. See #10357.