Binding failed on DependencyProperty of nullable types

[AlexanderEgorov]

Describe the bug
See the attached sample.
The following property cannot be bound with a two-way binding.

        public bool? IsChecked { get { return (bool?)GetValue(IsCheckedProperty); } set { SetValue(IsCheckedProperty, value); } }
        public static readonly DependencyProperty IsCheckedProperty =
            DependencyProperty.Register("IsChecked", typeof(bool?), typeof(MyControl), 
                new PropertyMetadata(null, (d, e) => ((MyControl)d).OnIsCheckedChanged()));
        void OnIsCheckedChanged() {
        }

Steps to reproduce the bug

1. Run the attached sample
2. Click on a button
3. Check an error in output: "Error: Converter failed to convert value of type 'Windows.Foundation.IReference1<Boolean>' to type 'IReference1'; BindingExpression: Path='IsChecked' DataItem='App1.MyControl'; target element is 'Microsoft.UI.Xaml.Controls.Primitives.ToggleButton' (Name='null'); target property is 'IsChecked' (type 'Boolean')."

Expected behavior
The binding should work without errors.

Version Info
[WinUI 3 - Project Reunion 0.5: 0.5.6]

Windows app type:

UWP	Win32
	Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

[StephenLPeters]

@teaP and @MikeHillberg (or maybe @jevansaks) do you recall the issues/work around here? similar to the issues with NumberBox's value property.

[teaP]

@teaP and @MikeHillberg (or maybe @jevansaks) do you recall the issues/work around here? similar to the issues with NumberBox's value property.

This isn't the same -- NumberBox.Value isn't nullable, it uses NaN as the unset value (which definitely caused OTHER binding issues because NaN != NaN).

[MikeHillberg]

But IIRC we wanted to use nullable for NumberBox, but couldn't because of this problem. I think there's an open Issue on this, that nullable isn't generally supported in binding and/or some markup scenarios, except for built-in WUX types.

We've worked around it for double types using NaN, int types using -1 or one of the infinities. I'm not sure how to work around it for a bool unfortunately; it doesn't have any magic values.

[AndrewKeepCoding]

Is there any progress or workaround on this?

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment or this will be closed in 5 days.