Strict mode - "Cannot invoke an object which is possibly 'undefined'" even if tested

[xthezealot]

TypeScript version:
2.7.2

Code (playground link, use strictNullChecks):

interface IWithOptionalMethod {
    om?(): any;
}

interface IFoobar {
    bar: { [index: string]: IWithOptionalMethod };
}

const foo: IFoobar = {
    bar: {},
};

// FAILS
Object.keys(foo.bar).forEach((key) => {
    if (foo.bar[key].om) {
        foo.bar[key].om(); // Cannot invoke an object which is possibly 'undefined'.
    }
});

// WORKS
Object.keys(foo.bar).forEach((key) => {
    let x = foo.bar[key].om;
    if (x) {
        x(); // OK
    }
});

Expected behavior:
No warning: foo.bar[key].omis tested and cannot be undefined (unless I missed something).

Actual behavior:
Warning: Cannot invoke an object which is possibly 'undefined'.

[RyanCavanaugh]

Duplicate #10530

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.