[Proposal] Support generic parameters as covariant return types in overrides

[just-ero]

Today, the below code is valid:

class C;
class D : C;

abstract class A
{
  public abstract C M();
}

class B : A
{
  public override D M() { }
}

However, this is currently not allowed:

class C;

abstract class A
{
  public abstract C M();
}

class B<T> : A
  where T : C
{
  public override T M() { }
}

Is this a limitation with how variance for generic parameters on classes works? Or would it be possible to enable this pattern?

[bernd5]

It actually compiles:

new B<C>();

class C
{ }

abstract class A
{
  public abstract C M();
}

class B<T> : A
  where T : C
{
  public override T M() { throw null; }
}

https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AXEUCuA7AHzxgHcACAIQB4BhAPgAoBKAbgFgAoTgAQCYyanAN5kAvp04BDYAGcMUSWAxk+ZAILDOZFQGYy0uQqUCyAWWbsO4rh1XUAKnTIh1WsiQAWMWGXvOBmhza3HoQAG7eUACWACYwvmbMZCIYHlAQ5Hg4ADbZLGKcokA==

[just-ero]

You're right, my example was incorrect. When C is an interface instead, the code no longer compiles:

interface I;

abstract class A
{
  public abstract I M();
}

class B<T> : A
  where T : I
{
  public override T M() { throw null; }
  //                ^
  // return type must be 'I' to match overridden member 'A.M()'
}

[ufcpp]

interface I;

abstract class A
{
    public abstract I M();
}

class B<T> : A
  where T : class, I // T must be a reference type
{
    public override T M() { throw null; }
}

[just-ero]

Sorry for the trouble. This makes so much sense.