Interface Overload Generic Member - Compiler error when removing overload

Bug Report

Removing an overload member Controller.handle from Controller causes a compilation error.

🔎 Search Terms

• Interface overload
• Member Type Parameter
• Works When Removing overload

⏯ Playground Link

Playground

💻 Code

Code below compiles

export interface Controller {
  handle<T>(httpRequest: HttpRequest<T>): HttpResponse<T>;
  handle<TRequest, TResponse>(httpRequest: HttpRequest<TRequest>): HttpResponse<TResponse>;
}

Code below does not compile

export interface Controller {
  // handle<T>(httpRequest: HttpRequest<T>): HttpResponse<T>;
  handle<TRequest, TResponse>(httpRequest: HttpRequest<TRequest>): HttpResponse<TResponse>;
}

Code below does not compile

export interface Controller {
  handle<T>(httpRequest: HttpRequest<T>): HttpResponse<T>;
  // handle<TRequest, TResponse>(httpRequest: HttpRequest<TRequest>): HttpResponse<TResponse>;
}

🙁 Actual behavior

Code does not compile when removing any of the overloads.

🙂 Expected behavior

Since having an overload in Controller.handle works, having just one member with the correct signature should also work.

Additional Info

• Stack Overflow Question

[MartinJohns]

Your interface says: This interface provides this generic method.
Your class implements no such generic method.

Simplified, you try this:

interface A { handle<T>(): T }
class E implements A { handle(): string { return "abc"; } }

The interface allows me to call handle with arbitrary generic arguments. The class ignores this, and just returns a string. So when calling the method I end up with nonsense:

const a: A = new E();
const r: number = a.handle<number>(); // Actually a string

When adding an overload to this example it will compile just fine as well, like your example. Playground link

[RyanCavanaugh]

I don’t recall offhand what the assignability rules are around overload targets. I would have said “assignable to each” but that’s clearly not what’s going on.