Symbol.hasInstance is not used in instanceof lookup

[wcandillon]

The following example returns true on v8:

class Foo {
  static [Symbol.hasInstance](instance) {
    return instance.__brand === "foo";
  }
}

console.log({ __brand: "foo" } instanceof Foo);

But returns false on Hermes.

The following example has the exact same issue as well:

function Foo() {}

Object.defineProperty(Foo, Symbol.hasInstance, {
  value: function (instance) {
    return instance.__brand === "foo";
  },
});

console.log({ __brand: "foo" } instanceof Foo);

I did find the following from the tests:

hermes/test/hermes/hasinstance.js

Line 13 in de90d05

C[Symbol.hasInstance] = function(o) {

But it looks like the examples above are not compliant?

[tmikov]

Thanks for reporting this. I just verified that it appears broken in Hermes, but works correctly in Static Hermes.

I will look into identifying and back-porting the fix to Hermes (which then will have to be cherry picked by RN). Our hope is that pretty soon everyone would simply be using Static Hermes as a drop-in replacement for Hermes.

[avp]

This was due to an incorrect fast path that we removed in Static Hermes. We just ported the fix to Hermes in f60d2b8