"property `prototype` is missing in statics of ..."

[adrians5j]

Please check the following code:

class BaseClass {
}

const checkIfCorrectClass___WORKS = (classToCheck: Class<BaseClass>):boolean => {
    return classToCheck.prototype instanceof BaseClass;
};

const checkIfCorrectClass___DOES_NOT_WORK = (classToCheck: Class<BaseClass>): void => {
    if (classToCheck.prototype instanceof BaseClass) {
        return;
    }

    throw Error(`Invalid class received.`);
};

Receiving following error:

Here's the snippet:

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjGBDAzrsAITwFMBhHfMAb1QF90M4A7XAFzAwAsSMBrAJJQycAE6jebCnlwB9AOoB5AEoBpAMpgAvGAAUWGQBU4ZHvwBcYafgA8xXOUq4AfAEpzAIzhwYJbM21nGlQwULAJNgBXUQCDfGNTXj4AOgAHUTg2TIBPVJIwAEtWNn8MEjgoIlJrXABuenrMFnZOM0FhMQkMKSdZABFFAFF1WQA5RUMFFVVtPTjcBLbLGrtqpzdLADc4AoATQOCwwsr9J0WktIystlz8ovZS8sr7RxlXQ6OwiOjmeqOGEJhNhcDJIQbiMS6AAGAmYm2wMD2nCc4V4JAKmxIu2SUNc9TotSAA

Could this be a bug maybe?

[moll]

Stumbled upon the same thing. It's a peculiar discrepancy indeed.

[gnprice]

Definitely a bug. Here's a smaller repro case:

(p) => p === Object.prototype;               // works

(p) => (p === Object.prototype) && true;     // ERROR

So the conditions seem to be related to the ….prototype expression appearing inside a conditional.

Pulling the ….prototype property access out into a variable also prevents the bug from happening:

const Object_prototype = Object.prototype;
(p) => (p === Object_prototype) && true;     // works

(p) => {
  const Object_prototype = Object.prototype;
  return (p === Object_prototype) && true;   // works
}

So that's potentially a clue for debugging... and is definitely also a workaround.

Playground link for the above examples. Tested on master and on v0.141.0, the latest.

From a quick bisection on the playground, the bug was introduced in v0.68.0. So that may also be a clue.

---

Demonstrating the workaround -- this variation on the example in the original report works fine:

const checkIfCorrectClass = (classToCheck: Class<BaseClass>): void => {
    const classToCheck_prototype = classToCheck.prototype;
    if (classToCheck_prototype instanceof BaseClass) {
        return;
    }

    throw Error(`Invalid class received.`);
};

Looks silly, but fortunately it's pretty easy to do.