Merge inherited interface methods according to variance

[alrz]

In the following code public void M(object arg) should be enough to implement the interface but neither the compiler nor the runtime accept such code and an explicit impl is required to bridge the two overloads. (sharplab.io)

interface ISomeInterface<in T> {
  void M(T arg);
}

interface IBase : ISomeInterface<object>;

interface IDerived : IBase, ISomeInterface<string> {
   // uncomment to resolve the error but this shouldn't be necessary
   // void ISomeInterface<string>.M(string obj) => M(obj as object); // calls base
}

class C : IDerived {
  // error: method `M(string)` is not implemented
  public void M(object arg) { }
}

[CyrusNajmabadi]

i thnik this would take a language change. i don't think the variance part of the lang spec supports this. The variance part affects things like conversions, but not interface impls.

[alrz]

Indeed, but since identical method signatures from different interfaces are already merged, I think this would be just a relaxation of that rule in both sides. On the runtime, the variance already guaranteed that the call is safe.

[CyrusNajmabadi]

I'm not opposed to this being allowed. But would need runtime to push for wanting this. Otherwise, it's unlikely we would do it on the lang side.