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.

リンクされた TypeScript Playground の再現コードから始め、Base::test と Derived::test に対して推論される型を比較します。オーバーライドされた ES6 メソッドがリテラルの戻り値型をどのように推論するかを追跡し、その後、リテラル型を失うことなく、派生関数が基底のリテラルユニオンに対して受け入れられることを確認します。