I work with TypeScript fairly closely and was surprised at your example, because I was sure that this already works. It turns out the type narrowing is actuall

I work with TypeScript fairly closely and was surprised at your example, because I was sure that this already works. It turns out the type narrowing is actually special-cased on only some particular AST. You can see it working here, where "charAt" would not be allowed if it was still |SomeInterface: http://www.typescriptlang.org/play/#src=interface%20SomeInterface%20%7Bx%3A%20number%7D%0Afunction%20doStuff(thing%20%3A%20string%20%7C%20SomeInterface)%20%3A%20void%20%7B%0A%20%20%20%20%20%20%20%20if%20(typeof%20thing%20%3D%3D%3D%20%22string%22)%20%7B%0A%09%09%09thing.charAt(0)%3B%0A%20%20%20%20%20%20%20%20%7D%0A%20%20%20%20%7D%0A%7D http://www.typescriptlang.org/play/#src=interface%20SomeInte...