XAML compiler generates invalid C++/WinRT code for bound hstring property

[sperrat]

Describe the bug

The code generated to read an hstring property in an x:Bind is missing boxing, and is causing the following error:
error C2679: binary '=': no operator found which takes a right-hand operand of type 'winrt::hstring' (or there is no acceptable conversion)

Steps to reproduce the bug

1. Create a C++/WinRT UWP project
2. Define a page and a view model with this IDL:

interface IViewModel
{
    String Name{ get; };
};

[bindable]
[default_interface]
runtimeclass ViewModel : IViewModel
{
    ViewModel();
}

// MainPage.idl
[default_interface]
runtimeclass MainPage : Windows.UI.Xaml.Controls.Page
{
    MainPage();

    static Windows.UI.Xaml.DependencyProperty ViewModelProperty{ get; };
    IViewModel ViewModel;
    Boolean IsReady{ get; };
}

3. Implement the corresponding C++ code
4. Add the following xaml; make sure x:Load is present, the error doesn't appear without it:

<!--  MainPage.xaml  -->
<Button
    x:Name="myButton"
    x:Load="{x:Bind IsReady, Mode=OneWay}"
    ToolTipService.ToolTip="{x:Bind ViewModel.Name, Mode=OneWay}">
    Click Me
</Button>

5. Compile the project; compilation will fail with
   ...\Generated Files\MainPage.xaml.g.hpp(259,21): error C2679: binary '=': no operator found which takes a right-hand operand of type 'winrt::hstring' (or there is no acceptable conversion)
   at

void Update_ViewModel_Name(::winrt::hstring obj, int32_t phase)
{
    if((phase & ((1 << 0) | NOT_PHASED | DATA_CHANGED)) != 0)
    {
        // MainPage.xaml line 14
        if (obj2)
        {
            // obj is properly boxed here
            Set_Windows_UI_Xaml_Controls_ToolTipService_ToolTip(obj2, ::winrt::box_value(obj), std::nullopt);
        }
        else
        {
            // obj is not boxed here
            obj2ToolTipDeferredValue = obj;
        }
    }
}

Expected behavior

This code compiles the string property is properly bound.

Screenshots

No response

NuGet package version

None

Windows version

Windows 11 (22H2): Build 22621, Windows 11 (21H2): Build 22000

Additional context

Workarounds:

1. The issue doesn't repro without x:Load. If using Visibility instead is acceptable, the code will compile and work.
2. Manually box the string property:

// BindingHelpers.idl
[default_interface]
static runtimeclass BindingHelpers
{
    static IInspectable ToObject(String value);
}

<!--  MainPage.xaml  -->
<Button
    x:Name="myButton"
    x:Load="{x:Bind IsReady, Mode=OneWay}"
    ToolTipService.ToolTip="{x:Bind local:BindingHelpers.ToObject(ViewModel.Name), Mode=OneWay}">
    Click Me
</Button>

[bpulliam]

Does this also repro in WinUI 3?

[sperrat]

Indeed; for the repro in WinUI3, take the same view model code as above, and create a custom control to hold the dependency property:

// CustomControl.idl
[default_interface]
runtimeclass CustomControl : Microsoft.UI.Xaml.Controls.Control
{
    CustomControl();

    static Microsoft.UI.Xaml.DependencyProperty ViewModelProperty{ get; };
    IViewModel ViewModel;
}

Then use this control in MainWindow.xaml:

<!-- MainWindow.xaml -->
<local:CustomControl
    x:Name="customControl"
    x:Load="{x:Bind customControl.ViewModel.IsReady, Mode=OneWay}"
    ToolTipService.ToolTip="{x:Bind customControl.ViewModel.Name, Mode=OneWay}" />

The same error will be raised in MainWindow.xaml.g.hpp
MainWindow.xaml.g.hpp(297,21): error C2679: binary '=': no operator found which takes a right-hand operand of type 'winrt::hstring'

[JaiganeshKumaran]

Duplicate of neglected old issue: #4787.