No warning for a class implementing itself

Bug Report

🔎 Search Terms

[implements itself, self implements]

🕗 Version & Regression Information

• This is the behavior in every version I tried from 3.3 on, and I reviewed the FAQ for entries about inheritance.

⏯ Playground Link

Playground link with relevant code

💻 Code

class Foo implements Foo {}

🙁 Actual behavior

This code passed typechecking.

🙂 Expected behavior

I expected the TypeScript compiler to emit an error on this code.

As far as I know, it's a no-op for a class to implement itself. The code author very likely intended to write something else.

This came up in the wild in TypeScript inside Google's repo. A file was trying to create a fake version of MediaSession with export class MediaSession implements MediaSession { for testing. The class accidentally implemented itself instead of the MediaSession interface from https://github.com/DefinitelyTyped/DefinitelyTyped/blob/c6f1fc8c0b7c2ebe8e099586af618b0a33cfef65/types/wicg-mediasession/index.d.ts#L20.

Another example found via Google: https://stackoverflow.com/questions/54306190/what-does-it-mean-when-a-class-implements-itself-in-typescript

Reproduce the issue from the linked TypeScript Playground using class Foo implements Foo {} and inspect how the compiler handles the implements clause. Add a regression test for the self-implementation case and verify that typechecking emits an error while valid implements clauses remain unaffected.