Add a DataType property on CompiledBindingExtension to enable specifying the start type explicitly on a binding.

Fixes #6158

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlBindingPathTransformer.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using XamlX.Ast;
 using XamlX.Transform;
+using XamlX.Transform.Transformers;
 using XamlX.TypeSystem;
 
 namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
@@ -15,7 +16,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             {
                 IXamlType startType = null;
                 var sourceProperty = binding.Children.OfType<XamlPropertyAssignmentNode>().FirstOrDefault(c => c.Property.Name == "Source");
-                if ((sourceProperty?.Values.Count ?? 0) == 1)
+                var dataTypeProperty = binding.Children.OfType<XamlPropertyAssignmentNode>().FirstOrDefault(c => c.Property.Name == "DataType");
+                if (sourceProperty?.Values.Count is 1)
                 {
                     var sourceValue = sourceProperty.Values[0];
                     switch (sourceValue)
@@ -99,6 +101,11 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                     }
                 }
 
+                if (dataTypeProperty?.Values.Count is 1 && dataTypeProperty.Values[0] is XamlAstTextNode text)
+                {
+                    startType = TypeReferenceResolver.ResolveType(context, text.Text, isMarkupExtension: false, text, strict: true).Type;
+                }
+
                 Func<IXamlType> startTypeResolver = startType is not null ? () => startType : () =>
                 {
                     var parentDataContextNode = context.ParentNodes().OfType<AvaloniaXamlIlDataContextTypeMetadataNode>().FirstOrDefault();

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindingExtension.cs

@@ -71,5 +71,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
         public CompiledBindingPath Path { get; set; }
 
         public object Source { get; set; }
+
+        public Type DataType { get; set; }
     }
 }