instanceof does not narrow when the class is intersected with &

[ehmicky]

🔎 Search Terms

instanceof, narrow.

🕗 Version & Regression Information

• This is a crash
• This is the behavior in every version I tried (from 3.8.3 to to 4.9.0-dev.20220919), and I reviewed the FAQ for entries about it.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface InstanceOne {
  one(): void
}
interface InstanceTwo {
  two(): void
}
const instance = {} as InstanceOne | InstanceTwo

const ClassOneWrong = {} as { new (): InstanceOne } & { foo: true }
if (instance instanceof ClassOneWrong) {
  instance.one() // Property 'one' does not exist, because `instance` is not narrowed down
}

const ClassOneRight = {} as { new (): InstanceOne, foo: true }
if (instance instanceof ClassOneRight) {
  instance.one() // Property 'one' exists and `instance` is narrowed down
}

🙁 Actual behavior

instanceof does not narrow down the left side when the right side is a type using &.

🙂 Expected behavior

It should narrow it down to InstanceOne, and instance.one() should pass.

[ahejlsberg]

Another instance of the same issue:

class A { a = 0 }
class B { b = 0 }

function f10(obj: A & { x: string } | B) {
    if (obj instanceof Object) {
        obj;  // B, but should be A & { x: string} | B
    }
    else {
        obj;  // A & { x: string }, but should be never
    }
}

[ehmicky]

Thanks for the fix @ahejlsberg! ❤️