Check inferred this parameter against interface requirement this: void

[jtbandes]

Bug Report

🔎 Search Terms

interface method requirement this: void

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo {
  method(this: void): number;
}
class Example implements Foo {
  x = 3;
  method() { return this.x; } // no error
}

🙁 Actual behavior

No error

🙂 Expected behavior

Error because the method implementation's implicit this: Example is not compatible with the interface requirement this: void.

The @typescript-eslint/unbound-method lint rule uses this: void as an annotation indicating the method is allowed to be used without binding this. However, there seems to be no checking from the TS compiler as to whether an inferred this type in a method implementation matches an interface requirement. Thus the lint rule can help you add this: void to the interface if you intend for functions to be usable when not bound... but neither the linter nor compiler currently help prevent runtime errors resulting from implementations with mismatched this types.

[sandersn]

As I recall, the implicit this for methods is still this: any not this: this, which was an intentional decision for backward compatibility. It might be time to revisit this decision. I'll investigate during the 4.4 beta.

[fatcerberus]

IIRC, I think I recall @RyanCavanaugh saying the reason for that is because this: this being the default implicitly makes all methods generic, which was a performance concern.

[fatcerberus]

Found it: #35451 (comment)

[sandersn]

From my memory of method instantiation, that's not true; classes are already all generic, so methods shouldn't need an additional type parameter (or instantiation) to use the classes' this.

That should be the first thing I investigate because I could easily be wrong.

[sandersn]

Yes, the only difference between this: Example and this: this in the above example is which type you get. The checker creates this<Example> at the same time it creates Example.

The next question is what breaks when methods have this: this by default.

Edit: No, I am wrong. The number of types is the same, but the method symbol also gets instantiated when there's a usage of a this type. It might be worth seeing how bad performance is nonetheless.