Added repro of #2725 to BindingDemo.

samples/BindingDemo/EnumToEnumerable.cs

@@ -0,0 +1,22 @@
+using System;
+using Avalonia.Markup.Xaml;
+
+namespace BindingDemo
+{
+    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);
+        }
+    }
+}

samples/BindingDemo/MainWindow.xaml

@@ -4,7 +4,7 @@
         xmlns:vm="clr-namespace:BindingDemo.ViewModels" 
         xmlns:local="clr-namespace:BindingDemo"
         Title="AvaloniaUI Bindings Test"
-        Width="800"
+        Width="1000"
         Height="600"
         x:DataType="vm:MainWindowViewModel">
   <Window.Styles>
@@ -40,6 +40,10 @@
             <CheckBox IsChecked="{Binding !BooleanString}">!BooleanString</CheckBox>
             <CheckBox IsChecked="{Binding !!BooleanString}">!!BooleanString</CheckBox>
           </StackPanel>
+          <StackPanel Margin="18" Spacing="4" Width="200">
+            <TextBlock FontSize="16" Text="Markup Extensions"/>
+            <ComboBox Items="{local:EnumToEnumerable {x:Type vm:MainWindowViewModel+NestedEnum}}" />
+          </StackPanel>
         </StackPanel>
         <StackPanel Orientation="Horizontal">
           <StackPanel Margin="18" Spacing="4" Width="200" HorizontalAlignment="Left">

samples/BindingDemo/ViewModels/MainWindowViewModel.cs

@@ -115,5 +115,11 @@ namespace BindingDemo.ViewModels
         {
             return BooleanFlag;
         }
+
+        public enum NestedEnum
+        {
+            Nested,
+            Enum,
+        }
     }
 }