instanceof should error when all constituents of a union are primitives

[DanielRosenwasser]

function foo(x: number | string) {
    if (x instanceof Number) {
        x
    }
}

Expected:

The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter

Actual:

No error, incorrect narrowing behavior (#14426)

Edit: Corrected the example to actually be a union.

[ajafff]

That's not always the expected behavior. When targeting a runtime with Symbol.hasInstance this should be allowed:

class PrimitiveNumber {
    static [Symbol.hasInstance](x) {
        return typeof x === 'number';
    }
}
console.log(123 instanceof PrimitiveNumber); // true

[DanielRosenwasser]

Sure, but

1. Most people aren't doing that.
2. We already error for 10 instanceof Number

[aluanhaddad]

Wouldn't an appropriate behavior then be to issue an error unless the value on the right hand side has a [Symbol.hasInstance] member taking a parameter of a type to which the type of value on the left hand side is assignable?

[DanielRosenwasser]

Something like #17360? Ironically, I think I've brought it up with @mhegazy in the past.

[charlespierce]

PR #19063 opened to resolve this issue.