Cannot specify that a property is inherited when defining interface

[vasyop]

Bug Report

🔎 Search Terms

interface inherited property

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

interface IObj {
    doSmth(): void;
}

class Obj implements IObj {
    public doSmth() {
        console.log('smth');
    }
}

const o: IObj = new Obj();

const clone = { ...o };
clone.doSmth(); // Uncaught TypeError: clone.doSmth is not a function

🙁 Actual behavior

no tsc compiler error

🙂 Expected behavior

I expect const o: IObj = new Obj(); to generate an error because doSmth is not an own enumerable property of o (it is inherited from the prototype). The spread operator seems to assume that doSmth is an own property of o, so, as a result, clone's type is { doSmth(): void; } but the "actual" type is {}.

If we have

const o = new Obj();

instead of

const o: IObj = new Obj();

we get a Property 'doSmth' does not exist on type '{}' error which indicates that IObj and Obj are clearly not compatible.
Perhaps we should be able to mark doSmth(): void; as "inherited"?

[RyanCavanaugh]

#9726

[vasyop]

I ran into this after using the spread operator on a Promise and the later calling .then() on it, which is apparently fine at compile time, but fails at runtime.
TS behaved as if the type of { ...somePromise } is Promise which is simply wrong.
I can imagine easily running into similar issues with other standard built-in objects.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.