"Overriding / Implementing Nullability"

public interface ITest {
	public string? GetStr();
	public void SetStr(string? s);

	public string GetStrNonNull();
	public void SetStrNonNull(string s);
}

public class Test : ITest {
	public string GetStr() => "Hello"; // Fine!
	public void SetStr(string s) { } // Warning here because you can not remove nullability on an input (i.e. parameter)

	public string? GetStrNonNull() => null; // Warning here because you can not add nullability on an output (i.e. return value)
	public void SetStrNonNull(string? s) { } // Fine!
}