Try in the playground.

class Dog {
}

declare let x: Dog | string;

if (x instanceof Dog) {
    x
}
else {
    x.toLowerCase()
}

Expected: No error, first branch's x has type Dog, second branch's x has type string
Actual: Property 'toLowerCase' does not exist on type 'never'. (first branch is string | Dog, second is never)

We should be accounting for primitives a little less sloppily. instanceof checks don't work on primitive types.