Generic interface inheritance conflict with extension method

I write 2 class inherit each other and each implement IEnumerable with difference generic type

Like this

public class A : IEnumerable<object>
{
    public IEnumerator<object> GetEnumerator()
    {
        throw new NotImplementedException();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

public class B : A, IEnumerable<string>
{
    public new IEnumerator<string> GetEnumerator()
    {
        throw new NotImplementedException();
    }
}

////

	static void Test()
	{
		var a = new A();
		var b = new B();

		a.Select((item,i) => i);
		b.Select((item,i) => i);
	}

Instance from A work fine but B cannot use all generic function in System.Linq

It seem B was seen by compiler that it is just IEnumerable but not IEnumerable<string>. It still have Cast<> and OfType<> exposed