Behaviour of !! operator has changed in 11.1

[stogle]

Describe the bug

The behaviour of the double-negation operator (!!) has changed between 11.0.10 and 11.1.0-rc1.

Consider the binding {Binding !!Value}, where Value is null. In 11.0.10 this binding yields false. In 11.1.0-rc1 the same binding yields null. This is a breaking change for my application.

To Reproduce

MainWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:MultiBindingBug.ViewModels"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="MultiBindingBug.Views.MainWindow"
        x:DataType="vm:MainWindowViewModel">
    <StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Value1: " />
            <TextBlock Text="{Binding Value1}" />
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="!!Value1: " />
            <TextBlock Text="{Binding !!Value1}" />
        </StackPanel>

        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Value2: " />
            <TextBlock Text="{Binding Value2}" />
        </StackPanel>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="!!Value2: " />
            <TextBlock Text="{Binding !!Value2}" />
        </StackPanel>
    </StackPanel>
</Window>

MainWindowViewModel.cs

namespace MultiBindingBug.ViewModels;

public partial class MainWindowViewModel
{
    public object? Value1 { get; } = 0;
    public object? Value2 { get; } = null;
}

Steps to reproduce:

1. Run the application. Avalonia 11.1.0-rc1 displays:
   

Expected behavior

Avalonia 11.0.10 displays:

Avalonia version

11.1.0-rc1

OS

Windows

Additional context

No response

[maxkatz6]

"!!" wasn't a defined and documented feature before. I would recommend using more explicit ObjectConverters.IsNotNull.

[stogle]

"!!" wasn't a defined and documented feature before. I would recommend using more explicit ObjectConverters.IsNotNull.

It was documented. From the link I posted above:

"You can also use the negation operator twice. For example, where you want to perform the conversion from integer to Boolean, and then negate that value."

This is not equivalent to any of the ObjectConverters.

[maxkatz6]

It was documented

TIL, thanks. Either way, I agree that it's a regression.

This is not equivalent to any of the ObjectConverters.

True, I was referring specifically to the "null to boolean" conversion.

[grokys]

Definitely looks like a regression introduced by the binding system refactor, will investigate.

Not sure if this is a showstopper and needs to be fixed for 11.0.0 or whether it can wait for 11.0.1 though.

[grokys]

We've decided that this issue is a serious enough regression that it necessitates an rc2.