[BUG] Unexpected Null-Forgiving Operators

[madbranch]

Describe the bug
Since version 14.1.1, it looks like we are forced to add a lot of unnecessary ! and ? when nullable reference types are enabled.

Steps To Reproduce
Here is an example of what we have to do to get a view model to compile and expose properties as non-null:

using ReactiveUI;

namespace Example
{
  public class SomeViewModel : ReactiveObject
  {
    public SomeViewModel()
    {
      m_MyCombinedText = this
        .WhenAnyValue(vm => vm.MyFirstText,
                      vm => vm.MySecondText,
                      (first, second) => first! + second!)
        .ToProperty(this, x => x.MyCombinedText);
    }

    public string MyFirstText
    {
      get => m_MyFirstText;
      set => this.RaiseAndSetIfChanged(ref m_MyFirstText, value);
    }

    public string MySecondText
    {
      get => m_MySecondText;
      set => this.RaiseAndSetIfChanged(ref m_MySecondText, value);
    }

    public string MyCombinedText => m_MyCombinedText.Value!;

    private string m_MyFirstText = string.Empty;
    private string m_MySecondText = string.Empty;
    private readonly ObservableAsPropertyHelper<string?> m_MyCombinedText;
  }
}

• WhenAnyValue's third parameter implementation shouldn't need the two !.
• The MyCombinedText property getter shouldn't need that !.
• The m_MyCombinedText private field type shouldn't need that ?.

Expected behavior
I would expect to be able to write the same SomeViewModel this way (still with nullable reference types enabled):

using ReactiveUI;

namespace Example
{
  public class SomeViewModel : ReactiveObject
  {
    public SomeViewModel()
    {
      m_MyCombinedText = this
        .WhenAnyValue(vm => vm.MyFirstText,
                      vm => vm.MySecondText,
                      (first, second) => first + second)
        .ToProperty(this, x => x.MyCombinedText);
    }

    public string MyFirstText
    {
      get => m_MyFirstText;
      set => this.RaiseAndSetIfChanged(ref m_MyFirstText, value);
    }

    public string MySecondText
    {
      get => m_MySecondText;
      set => this.RaiseAndSetIfChanged(ref m_MySecondText, value);
    }

    public string MyCombinedText => m_MyCombinedText.Value;

    private string m_MyFirstText = string.Empty;
    private string m_MySecondText = string.Empty;
    private readonly ObservableAsPropertyHelper<string> m_MyCombinedText;
  }
}

Environment

• OS: Windows
• Version 10
• Device: PC
• ReactiveUI 14.1.1

Additional context
The expected behavior used to work before 14.1.1. It seems to be related ot #2759

[glennawatson]

Don't believe this is a bug. Eg WhenAnyValue would emit null values so you should be handling them more appropriately and not just use !

[madbranch]

Why would it give a null if everything I give it is never null? Why would I need to handle an impossible case?

[madbranch]

Putting nullable types ? types everywhere defeats the purpose of enabling nullable reference types. I don't understand how this is not a breaking change.

[madbranch]

I would like to understand what would be the expected implementation of that SomeViewModel instead of adding !?

I don't see how handling nulls would make it better. Why would they be null?

Thank you for your time.

[github-actions]

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