Interface implementation may have more-specific argument types than interface

[twavv]

Bug Report

🔎 Search Terms

interface, variance (covariance, contravariance), interface methods, implementation, soundness, unsoundness, narrowing, interface assignability

🕗 Version & Regression Information

Seems to effect every version of TypeScript I tried on the Playground (3.X and 4.X).

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.5.2#code/PTAEFkE9QYwewCYFNQHM5IM6gBZIE5IBQRAlgHYAuBAZgIYwoCqmBoA3kaN6OXQLZIAXKEyV8FVAG4iAXxIVq+eo1ABxQkiUcuaTZQAUmOgBsArpTqVScciLETyqADSgzrfAH4RLNgB9eM34AIwIASntxSRl5InhyMVAaOHx+Uw0kLQIRDKz8UABeHW5UfSNXd2zQX3ww4p4QHiamgD02lt1uRoBlHDgzEwReOEpQUNBTEzgAdyQEEQADGtAA8iDQ-ICzcmQaCjmF0AMafDh+dX1wztBGzD6BofIRsZQ6TExSVD5gkxRKOFASw8hzwhAAdNdCJQzPhyICACTsTCyUD8aDIOiERKIyr4MF8QSyBYyJqNZrkinNdrUtrXRoAdVIJhMoEoOFO0wmoHw22sglABFO+VCMDolTcHlRdGgDGhk2g422u32CDpYBSXLWIQIEO4smcchkcVsmDgvzBU1QxxSaRMuSUYNKmUMACI8My4C7XOxeAJhKAXQAVfB0ABupEwLtAsjCYRk8VN5st1tS6UueKdWgMbqQHpdsZkQA

💻 Code

// My code goes here

interface User {
    name: string;
}

interface Greeter {
  greet(salutation: string, user?: User | number): string;
}

const formalGreeter: Greeter = {
  greet(s, user: User) {
    //           ^^^^
    // Should not be allowed: `User | number | undefined` (from Greeter)
    // should not be assignable to `User` here.
    return `${s} my dearest ${user.name}`;
    //                        ^^^^^^^^^
    // Will throw a runtime error because user may actually be undefined
    // or a number.
  },
};

console.log(formalGreeter.greet("hello", { name: "Travis" }));
console.log(formalGreeter.greet("hello"));

🙁 Actual behavior

Should not typecheck (see comments above). The implementation given should not be assignable to Greeter.

🙂 Expected behavior

Typechecks successfully.

[RyanCavanaugh]

https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant

[twavv]

Thanks! I suspected it was probably something like that, just didn't quite know how to search for it. Hopefully this will help other people find the issue.

At the very least I wish this behavior would be revisited for "simple" types (i.e., scalars) since it effectively disables strict null checking when implementing a function. I discovered the issue when I did something like this:

Sentry.init({
  beforeSend(event: Sentry.Event, hint: Sentry.Hint) { ... }
})

since TS didn't warn me that the hint param can be undefined.

But feel free to close if that discussion is already been had or out of scope.

[fatcerberus]

This is a very commonly encountered issue, for what it's worth.

If you're able to use strictFunctionTypes in your project, then you can declare your methods as fields if you want them to be checked contravariantly. It's even enough just to do that in the interface definition, object literals assigned to it can still use method syntax:

https://www.typescriptlang.org/play?ts=4.5.2#code/PTAEFkE9QYwewCYFNQHM5IM6gBZIE5IBQRAlgHYAuBAZgIYwoCqmBoA3kaN6OXQLZIAXKEyV8FVAG4iAXxIVq+eo1ABxQkiUcuaTZREAKTHQA2AV0p1KpOORFiJ5VABpQ51vgD8IlmwA+vOb8AEYEAJSgALwAfKLikjLyRPDkYqA0cPj8ZhpIWgQieQX40TrcqPrGbh6FoH74kZw8oCAt7dwAet2dutxtAMo4cOamCLxwlKBhoGamcADuSAgiAAYNoIHkwWGlgebkyDQUy6ughjT4cPzq+hF9rWCYw6Pj5JPTKHSYmKSofCFTChKHBQOtPGc8IQAHQPQiUcz4chggAk7EwslA-GgyDohHSaNq+GhfEEslWMhabQ6NNpXR6DN6VLAAHVSKZTKBKDgrgtZqB8AcbIJQAQrqUwjA6LV3J4sXRoAwEXNoDMDkcTggHm0svztqECLDuLIXHIZCk7Jg4EDofNUBcsjlTMUlNDKvlKIYAER4DlwL1udi8ATCUBegAq+DoADdSJgvaBZOFwjJUlabXaHdlcndie6tN7ffMvcmZEA

[typescript-bot]

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