Added failing test for #8672.

tests/Avalonia.Markup.UnitTests/Data/TemplateBindingTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
 using Avalonia.Controls;
@@ -217,6 +218,37 @@ namespace Avalonia.Markup.UnitTests.Data
             }
         }
 
+        [Fact]
+        public void Should_Not_Pass_UnsetValue_To_MultiBinding_During_ApplyTemplate()
+        {
+            var converter = new MultiConverter();
+            var source = new Button
+            {
+                Content = "foo",
+                Template = new FuncControlTemplate<Button>((parent, _) =>
+                    new ContentPresenter
+                    {
+                        [~ContentPresenter.ContentProperty] = new MultiBinding
+                        {
+                            Converter = converter,
+                            Bindings =
+                            {
+                                new TemplateBinding(ContentControl.ContentProperty),
+                            }
+                        }
+                    }),
+            };
+
+            source.ApplyTemplate();
+
+            var target = (ContentPresenter)source.GetVisualChildren().Single();
+
+            // #8672 was caused by TemplateBinding passing "unset" to the MultiBinding during
+            // ApplyTemplate as the TemplatedParent property doesn't get setup until after the
+            // binding is initiated.
+            Assert.Equal(new[] { "foo" }, converter.Values);
+        }
+
         private class PrefixConverter : IValueConverter
         {
             public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
@@ -247,5 +279,16 @@ namespace Avalonia.Markup.UnitTests.Data
                 return null;
             }
         }
+
+        private class MultiConverter : IMultiValueConverter
+        {
+            public List<object> Values { get; } = new();
+
+            public object Convert(IList<object> values, Type targetType, object parameter, CultureInfo culture)
+            {
+                Values.AddRange(values);
+                return values.FirstOrDefault();
+            }
+        }
     }
 }