Can't make many templated controls AoT compatible

[dotMorten]

Describe the bug

If you're building custom controls, it is VERY hard to make them AoT compatible, because the control templates can no longer use Binding, and you can't use x:DataType.
Say you have a control that provides a default ItemTemplate, this item template must use x:Bind to work with AoT, but you can't use x:Bind in a resource template.
Trying to do so will give you the following error:

1>E:\source\Themes\Generic.xaml(21,36): XamlCompiler error WMC1119: This Xaml file must have a code-behind class to use {x:Bind}. See http://go.microsoft.com/fwlink/?LinkID=532920&clcid=0x409 for more information

Not using x:DataType will cause the app to crash when publishing trimmed because the datatype isn't preserved and setting the ItemSource will crash (with the unhelpful System.ArgumentException: 'Value does not fall within the expected range.' message on the ItemsSource setter.

Another issue that often is hit is getting access to a sub-property from template binding, that isn't possible. There's really no way to convert this classic binding expression:
Text="{Binding SomeProperty.SubProperty, RelativeSource={RelativeSource TemplatedParent}}" since you can't replace it with Text="{TemplateBinding SomeProperty.SubProperty}"

It feels to me that there's a major thing missing from the AoT story when it comes to custom control templating.

Steps to reproduce the bug

1. Unzip the following sample app: AoTWoes.zip
2. Try and compile
3. Observe the WMC1119 xaml compiler error.
4. In Themes\Generic.xaml remove the default item template and include the commented-out {Binding} based item template.
5. Run the app and observe runtime-crash
6. Disable PublishAoT and notice app runs just fine.

Expected behavior

A way to use x:Bind in control templates that is AoT compatible.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

Windows version

Windows 11 (24H2): Build 26100

Additional context

I also tried adding a codebehind file to Generic.xaml like this:

using Microsoft.UI.Xaml;

namespace AoTWoes;

public partial class ThemeResources : ResourceDictionary
{
    public ThemeResources()
    {
        InitializeComponent();
    }
}

And set the x:Class property on Generic.xaml. That gets past the issue of being able to use x:DataType on the data template, however it still doesn't work at runtime. It also has the downside of introducing public classes that really shouldn't be exposed to the end user.

[ghost1372]

It seems that they noticed this before

https://learn.microsoft.com/en-us/windows/apps/windows-app-sdk/stable-channel#resolving-aot-issues

[michael-hawker]

We've noticed similar issues with ElementName bindings as well in a control template, e.g. from the WCT ColorPicker:

 <controls:SwitchPresenter x:Name="ContentContainer" 
                           Grid.Row="2" 
                           Value="{Binding ElementName=ColorPanelSelector, Path=SelectedItem.Name}"> 

from CommunityToolkit/Windows#509 (comment)