Added passing tests for #2725.

Including a repro (`EnumToEnumerable`) from a customer. Looks like the problem can't be reproduced in unit tests.

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

@@ -0,0 +1,21 @@
+using System;
+
+namespace Avalonia.Markup.Xaml.UnitTests
+{
+    public class EnumToEnumerable : MarkupExtension
+    {
+        private readonly Type _enumType;
+
+        public EnumToEnumerable(Type type)
+        {
+            if (!type.IsEnum)
+                throw new ArgumentException(nameof(type));
+            _enumType = type;
+        }
+
+        public override object ProvideValue(IServiceProvider serviceProvider)
+        {
+            return Enum.GetValues(_enumType);
+        }
+    }
+}

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/MarkupExtensionTests.cs

@@ -0,0 +1,32 @@
+using System.Collections;
+using Avalonia.Controls;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
+{
+    public class MarkupExtensionTests : XamlTestBase
+    {
+        [Fact]
+        public void Markup_Extension_Can_Accept_Nested_Type()
+        {
+            using (UnitTestApplication.Start(TestServices.MockWindowingPlatform))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests;assembly=Avalonia.Markup.Xaml.UnitTests'>
+    <ListBox Items='{local:EnumToEnumerable {x:Type local:TestViewModel+NestedEnum}}'/>
+</Window>";
+
+                var window = AvaloniaRuntimeXamlLoader.Parse<Window>(xaml);
+                var listBox = (ListBox)window.Content;
+                var items = (IList)listBox.Items;
+
+                Assert.Equal(2, items.Count);
+                Assert.Equal(TestViewModel.NestedEnum.Item1, items[0]);
+                Assert.Equal(TestViewModel.NestedEnum.Item2, items[1]);
+            }
+        }
+    }
+}

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

@@ -37,5 +37,16 @@ public TestViewModel Child
                 RaisePropertyChanged();
             }
         }
+
+        public class NestedType
+        {
+            public string String { get; set; }
+        }
+
+        public enum NestedEnum
+        {
+            Item1,
+            Item2,
+        }
     }
-}
+}

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

@@ -1,5 +1,6 @@
 using Avalonia.Controls;
 using Avalonia.Controls.Presenters;
+using Avalonia.Markup.Xaml.Templates;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -132,5 +133,47 @@ public void Can_Set_DataContext_In_DataTemplate()
                 Assert.Same(viewModel.Child.Child, canvas.DataContext);
             }
         }
+
+        [Fact]
+        public void DataType_Can_Be_Nested_Type()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:sys='clr-namespace:System;assembly=netstandard'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests;assembly=Avalonia.Markup.Xaml.UnitTests'>
+    <Window.DataTemplates>
+        <DataTemplate DataType='{x:Type local:TestViewModel+NestedType}'>
+            <TextBlock Text='{Binding String}' Tag='NestedType'/>
+        </DataTemplate>
+    </Window.DataTemplates>
+    <ContentControl Name='target' Content='{Binding}'/>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var dataTemplate = (DataTemplate)window.DataTemplates[0];
+                var target = window.FindControl<ContentControl>("target");
+
+                window.DataContext = new TestViewModel.NestedType
+                {
+                    String = "Nested"
+                };
+
+                window.ApplyTemplate();
+                target.ApplyTemplate();
+                ((ContentPresenter)target.Presenter).UpdateChild();
+
+                Assert.Equal(typeof(TestViewModel.NestedType), dataTemplate.DataType);
+
+                var child = Assert.IsType<TextBlock>(target.Presenter.Child);
+                Assert.Equal("Nested", child.Text);
+                Assert.Equal("NestedType", child.Tag);
+            }
+        }
+
+        public class NestedType
+        {
+        }
     }
 }