JS ES6 class derived member functions typed with literals don't get assigned the correct type

Bug Report

In rare cases (such as when literals are used in a type constraint), ES6 class overridden functions will cause strange issues when returning said literals from the derived function.

🔎 Search Terms

ts es6 class derived

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about ES6 classes

⏯ Playground Link

Playground link with relevant code

💻 Code

class Base
{
    test()
    {
        if (Math.random() > 0.5)
            return 5;

        return false;
    }
}

class Derived extends Base
{
    // error here: for some reason, Derived::test() is typed as "() => boolean"
    // even though the literal false is being returned, and as such it says it can't
    // match Base::test
    test()
    {
        return false;
    }
}

🙁 Actual behavior

The derived function's auto-typing assumes it is the literals' actual type, such as number or boolean, instead of the literals themselves. Derived::test, if not overridden, is typed as () => false, which should match () => 5 | false, but because it's typed as boolean instead it breaks (this only seems to happen if we're overriding a base member function)

🙂 Expected behavior

The derived function should be able to match if the typing is handled as normal.

Inizia con la riproduzione collegata di TypeScript Playground e confronta i tipi inferiti per Base::test e Derived::test. Traccia il modo in cui i metodi ES6 sovrascritti inferiscono tipi di ritorno letterali, quindi verifica che la funzione derivata venga accettata rispetto all'unione letterale della base senza perdere i tipi letterali.