Instanceof requires callable or constructable right-hand side

[sandersn]

Fixes #12709

The check for the right-hand side of instanceof previously required that the type be any or a subtype of Function. This is too strict, and breaks if a class has some otherwise-unrelated functions like toString, bind, etc that don't match Function.toString, Function.bind, etc.

This PR loosens the check to allow a right-hand side type with a call or construct signature, even if the type isn't assignable to Function.

[sandersn]

@mhegazy do you want to take a look?

[mhegazy]

I would like to discuss this in the design meeting first. will put it on the agenda.

[DanielRosenwasser]

To get some discussion going, one of two requirements must be satisfied for instanceof to work in a reasonable way:

1. The object has an method whose key is Symbol.hasInstance.
2. If the object has no member keyed by Symbol.hasInstance, the object must be callable.

Check out the relevant section of the spec itself.

[DanielRosenwasser]

So in short, I think checking for some number of call/construct signatures is the right move - however, we could also check for a Symbol.hasInstance.

[mhegazy]

So in short, I think checking for some number of call/construct signatures is the right move - however, we could also check for a Symbol.hasInstance.

I would say we should add that check as well to be ES6 complaint.

[mhegazy]

as per #13331, let's get this one in.