this is return type of method isn't checked when implementing

[kermanx]

🔎 Search Terms

"this is type", "return type", "implement", "inherit", "not checked"

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.3#code/JYOwLgpgTgZghgYwgAgILIN4ChnILYRgAWA9gCYCMAFAJQBcyxwAzsi5jrsnAxZwL6cCxcgCZaDAEYkSAGwhwQAbiyCEsuM1YAhNngAO8guFbpsuAPQXkAdSIBPZCBLJoUElAD8QwqUq1MZEFOK2QAATBmAFoIAA99CAQwGKh3KGQAFXsE5AByADcSYDJctlZnMG4tYABzEDhJeUYXMGyUXOk5BRBcnxEycRpAwWDWnIzkAF5kACVCAFcoECyEgB4ASRBmMEUkFYhVsYgSGGRtAD4AbQAiYT8Ka4Bdc6wgA

💻 Code

interface A {
  method1(): this is {
    a: 1
  };
  method2(): boolean;
}
class B implements A {
  // Why no error?
  method1() { }

  // @ts-expect-error Type 'void' is not assignable to type 'boolean'
  method2() { }
}

🙁 Actual behavior

B.method1 produces no error, and its return type is void.

🙂 Expected behavior

B.method1 should be an error.

Additional information about the issue

If A in the above example is a class, and B is extending A, this bug will also happen.