So I started exploring this again today, and it ends up that what you had works, but if I parens on the typeof check, it no longer works: if (typeof(x) ===

So I started exploring this again today, and it ends up that what you had works, but if I parens on the typeof check, it no longer works: if (typeof(x) === "string") I opened an issue: https://github.com/Microsoft/TypeScript/issues/9391 https://github.com/Microsoft/TypeScript/issues/9391 Thanks for the help!