'instanceof' changes type outside of 'if' statement

TypeScript Version: 3.4.*

Search Terms:

instanceof

Code

// @strictNullChecks: true
interface OnChanges {
    onChanges(changes: Record<string, unknown>): void
}
interface Validator {
    validate(): null | Record<string, unknown>;
}

class C {
    validate() {
        return {}
    }
}
function foo() {
    let v: Validator & Partial<OnChanges> = null as any;
    if (v instanceof C) {
    }
    if (v.onChanges) { // error here
        v.onChanges({});
    }
}

Expected behavior:

No error.

Actual behavior:

Property 'onChanges' does not exist on type 'C | (Validator & Partial<OnChanges>)'.
  Property 'onChanges' does not exist on type 'C'.

instanceof shouldn’t change the type of the variable outside of the if statement.

This only happens with strictNullChecks enabled.

Playground Link: https://typescript-play.js.org/#code/JYOwLgpgTgZghgYwgAgPIgMIAs4gOYQDOyA3gFDKXID2mO+RAFAvQYQFzIBKEC1UAEwA8hMFFB4ANMgCuIANYhqAdxAA+AJScAbtWACyAXzKhIsRCgBqcADb64YfqQpVtt+5EZbkIGTZvIAD7cvPzCouL40nKKKuoA3EZkZAg2cITEGM5UyG52Ag4QXtk5VFAQYDJQIKTGOcbGMHIIYMC0yDDU1MXkOTYVuZzW+Q5OAGTIAApwUK22QujYuGxqyAC8Pn4B6ci4AJ6JOcAwyIzayKCiuEjUJxgaJZR1VMen2gB0tEsMhA+9pZQPl9WEwSIYNIcqA0gA

Related Issues:

[weswigham]

The correct fix is to make an intersection even if one of type or candidate is assignable to the other - unions only do subtype reduction, so anything that narrows to a type by assignability (rather than subtype) will have this issue.

[orta]

This fix was reverted with https://github.com/microsoft/TypeScript/pull/41849