Error messages are difficult to parse when class incorrectly implements an interface

[robertknight]

If a class incorrectly implements an interface which it explicitly claims to implement, the compiler produces "Argument of type '' is not assignable to parameter of type ''" at all the locations where an instance of the class is assigned to a variable or function parameter of the interface type.

This results in a lot of redundant error messages which obscure the root issue - the error message explaining why the class doesn't implement the interface.

I would suggest that if a class explicitly claims to implement an interface, that call-sites shouldn't issue errors about type incompatibilities.

eg. With this code:

interface WebServiceClient {
    getData(): string;
}

class WebServiceImpl implements WebServiceClient {
    getData(): number { // incorrect implementation
        return 0;
    }
}

function doSomethingWithWebService(client: WebServiceClient): void {
}

var webService = new WebServiceImpl();
doSomethingWithWebService(webService);

Don't produce the "Argument of type 'WebServiceImpl' is not assignable to parameter of type 'WebServiceClient'" error at the doSomethingWithWebService() call, only the error for the WebServiceImpl implementation.

[danquirk]

This would reduce some noise. I'm not sure the cost of doing it is worth what's gained. Are there other languages that do this sort of thing?

[RyanCavanaugh]

There's just not a clean way to implement this in the compiler today (relative the gain in error quality)