Type guard does not handle object properties

[Strate]

TypeScript Version:

actual version on http://www.typescriptlang.org/Playground

Code

// A self-contained demonstration of the problem follows...
interface Props {
    options: {
        name: string | boolean
    }
}

let v: Props

if (typeof v.options.name === "string") {
    v.options.name.indexOf("123")
}

Expected behavior:
No error

Actual behavior:
Error: Property "indexOf" does not exist on type "string | boolean"

[yortus]

Yet another duplicate of #3812. This issue is getting really popular. See also this comment.