Allow overrides with more specialized return types

[kingces95]

Proposal

Allow overrides with more specialized return types.

For example, I expect the CLR to allow the addition of a virtual method MethodBase GetGenericMethodDefinition() on MethodBase to be overriden by the existing MethodInfo GetGenericMethodDefinition() on the derived class MethodInfo even though the return type of the override is more specialized (see https://github.com/dotnet/corefx/issues/23763). In actuality, the signatures of virtual overrides must match exactly.

For example, I expect this this fiddle to compile (and run) but actually fails with compiler error CS0508 'MethodInfo.GetGenericTypeDefinition()': return type must be 'MethodBase' to match overridden member 'MethodBase.GetGenericTypeDefinition()'.

public class MethodBase {
    public virtual MethodBase GetGenericTypeDefinition() => null;
}

public class MethodInfo : MethodBase {
    public override MethodInfo GetGenericTypeDefinition() => null;
}

Rational

Java does it. Why haven't we?

CSharp Team On-board

See dotnet/roslyn#357
See https://github.com/dotnet/csharplang/blob/master/proposals/covariant-returns.md
See http://flylib.com/books/en/2.663.1.64/1/
See https://www.red-gate.com/simple-talk/blogs/implementing-method-override-covariance-on-c-part-2/