Readonly properties type narrowing doesn't flow into inner function scopes

TypeScript Version: 3.3.0-dev.20190105

Search Terms: Readonly, strictNullChecks, optional, inner function

Code

Using strictNullChecks: true:

interface Thing {
    foo?: (a: number) => boolean;
}

const t: Readonly<Thing> = {
    foo: (a) => true,
};

if (t.foo !== undefined) {
    const bar = () => {
        t.foo(5);
    }
}

Expected behavior:

👍 Compile

Actual behavior:

On the expression t.foo(5); the following error:

Cannot invoke an object which is possibly 'undefined'.
(property) foo?: ((a: number) => boolean) | undefined

Playground Link: https://www.typescriptlang.org/play/index.html#src=interface%20Thing%20%7B%0D%0A%20%20%20%20foo%3F%3A%20(a%3A%20number)%20%3D%3E%20boolean%3B%0D%0A%7D%0D%0A%0D%0Aconst%20t%3A%20Readonly%3CThing%3E%20%3D%20%7B%0D%0A%20%20%20%20foo%3A%20(a)%20%3D%3E%20true%2C%0D%0A%7D%3B%0D%0A%0D%0Aif%20(t.foo%20!%3D%3D%20undefined)%20%7B%0D%0A%20%20%20%20const%20bar%20%3D%20()%20%3D%3E%20%7B%0D%0A%20%20%20%20%20%20%20%20t.foo(5)%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0A

Related Issues:

• Possibly a duplicate https://github.com/Microsoft/TypeScript/issues/22137, but I wasn't sure.
• Following the theory of this issue, this should be seen as a bug: https://github.com/Microsoft/TypeScript/issues/10927.

Start with the supplied TypeScript reproduction and its Playground link, focusing on control-flow narrowing for the readonly optional property inside the arrow function. Confirm the reported error and compare the behavior with the related issues; done means the example compiles without incorrectly allowing an undefined call.