Types defining Symbol.hasInstance should be eligible to use with the instanceof operator

[lazytype]

TypeScript Version: 3.9.4

As specified here, https://tc39.es/ecma262/#sec-instanceofoperator, it should be sufficient for an object to define a function for the Symbol.hasInstance well-known symbol in order to be used as the right-hand side of the instanceof operator.

It seems like this was intended to be implemented based on these comments: #12728 (comment)
but for whatever reason that didn't happen it in the PR

Search Terms: Symbol.hasInstance

Code

({}) instanceof {[Symbol.hasInstance]: () => true}
// Running this in the browser evaluates to `true`

Expected behavior:
No type-errors

Actual behavior:

({}) instanceof {[Symbol.hasInstance]: () => true}
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2359: The right-hand side of an 'instanceof' expression must be of type 'any' or of a type assignable to the 'Function' interface type.

Playground Link: Playground Link

Related Issues:

[lazytype]

I have a tested and working branch that implements this proposal here: https://github.com/lazytype/TypeScript/commit/0c226a775f63f4dd97f6a65422707d4231787188

Holding off on creating a PR until this issue gets tagged with the "Backlog" milestone, per the contributor guidelines.

[ExE-Boss]

See also: #17360

[rotu]

The typescript compiler also spuriously complains when the lhs of instanceof is not an object:

class MyNumber {
  static [Symbol.hasInstance](x:any) {
    return (typeof x === 'number');
  }
}

console.log(1 instanceof MyNumber);

It gives this error:

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

[shrinktofit]

Why was this issue marked as "Suggestion"? I think it should be labeled as bug. @RyanCavanaugh

[MichaelMitchell-at]

I have a tested and working branch that implements this proposal here: lazytype@0c226a7

Holding off on creating a PR until this issue gets tagged with the "Backlog" milestone, per the contributor guidelines.

❤️ Thanks for finally fixing the bug @rbuckton. I can finally delete this branch.