Use style priority for attached values in templates.

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

@@ -15,12 +15,19 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         {
             var bindingPriorityType = context.GetAvaloniaTypes().BindingPriority;
 
+            // The node is a candidate for transformation if:
+            // - It's a property assignment to an Avalonia property
+            // - There's a ControlTemplate ancestor
+            // - There's just a direct call setter available
             if (node is XamlPropertyAssignmentNode prop &&
                 prop.Property is XamlIlAvaloniaProperty avaloniaProperty &&
-                context.ParentNodes().Any(IsControlTemplate))
+                context.ParentNodes().Any(IsControlTemplate) &&
+                prop.PossibleSetters.Count == 1 &&
+                prop.PossibleSetters[0] is XamlDirectCallPropertySetter)
             {
-                // If there are any setters which accept a binding priority, then add the Style
-                // binding priority as a value.
+                // Check if there are any setters on the property which accept a binding priority -
+                // this filters the candidates down to styled and attached properties. If so, then
+                // use this setter with BindingPriority.Style.
                 var setPriorityValueSetter =
                     avaloniaProperty.Setters.FirstOrDefault(x => x.Parameters[0] == bindingPriorityType);
                 
@@ -30,6 +37,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                 {
                     prop.PossibleSetters = new List<IXamlPropertySetter> { setPriorityValueSetter };
                     prop.Values.Insert(0, new XamlConstantNode(node, bindingPriorityType, (int)BindingPriority.Style));
+                    return node;
                 }
             }
 

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/XamlIlAvaloniaPropertyHelper.cs

@@ -190,7 +190,10 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
             if (!original.CustomAttributes.Any(ca => ca.Type.Equals(types.AssignBindingAttribute)))
                 Setters.Insert(0, new BindingSetter(types, original.DeclaringType, field));
 
-            if (field.FieldType.GenericTypeDefinition == types.StyledPropertyT)
+            // Styled and attached properties can be set with a BindingPriority when they're
+            // assigned in a ControlTemplate.
+            if (field.FieldType.GenericTypeDefinition == types.StyledPropertyT ||
+                field.FieldType.GenericTypeDefinition == types.AvaloniaAttachedPropertyT)
             {
                 var propertyType = field.FieldType.GenericArguments[0];
                 Setters.Insert(1, new SetValueWithPrioritySetter(types, original.DeclaringType, field, propertyType));

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/ControlTemplateTests.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
     public class ControlTemplateTests : XamlTestBase
     {
         [Fact]
-        public void Inline_ControlTemplate_Values_Are_Set_With_Style_Priority()
+        public void Inline_ControlTemplate_Styled_Values_Are_Set_With_Style_Priority()
         {
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
@@ -43,7 +43,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
         }
 
         [Fact]
-        public void Style_ControlTemplate_Values_Are_Set_With_Style_Priority()
+        public void Style_ControlTemplate_Styled_Values_Are_Set_With_Style_Priority()
         {
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
@@ -76,6 +76,68 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             }
         }
 
+        [Fact]
+        public void ControlTemplate_Attached_Values_Are_Set_With_Style_Priority()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Button>
+        <Button.Template>
+            <ControlTemplate>
+                <ContentPresenter Name='PART_ContentPresenter'
+                                  DockPanel.Dock='Top'/>
+            </ControlTemplate>
+        </Button.Template>
+    </Button>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var button = (Button)window.Content;
+
+                window.ApplyTemplate();
+                button.ApplyTemplate();
+
+                var presenter = (ContentPresenter)button.Presenter;
+                Assert.Equal(Dock.Top, DockPanel.GetDock(presenter));
+
+                var diagnostic = presenter.GetDiagnostic(DockPanel.DockProperty);
+                Assert.Equal(BindingPriority.Style, diagnostic.Priority);
+            }
+        }
+
+        [Fact]
+        public void ControlTemplate_TemplateBindings_Are_Set_With_TemplatedParent_Priority()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Button Content='Foo'>
+        <Button.Template>
+            <ControlTemplate>
+                <ContentPresenter Name='PART_ContentPresenter'
+                                  Content='{TemplateBinding Content}'/>
+            </ControlTemplate>
+        </Button.Template>
+    </Button>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var button = (Button)window.Content;
+
+                window.ApplyTemplate();
+                button.ApplyTemplate();
+
+                var presenter = (ContentPresenter)button.Presenter;
+                Assert.Equal("Foo", presenter.Content);
+
+                var diagnostic = presenter.GetDiagnostic(ContentPresenter.ContentProperty);
+                Assert.Equal(BindingPriority.TemplatedParent, diagnostic.Priority);
+            }
+        }
+
         [Fact]
         public void ControlTemplate_With_Nested_Child_Is_Operational()
         {