Make default binding Source = UnsetProperty.

Null is a theoretically valid value for `Source`; setting it to null shouldn't mean "use the data context".

src/Avalonia.Base/Data/Core/ExpressionNodes/DataContextNode.cs

@@ -5,6 +5,8 @@ namespace Avalonia.Data.Core.ExpressionNodes;
 
 internal class DataContextNode : ExpressionNode, ISourceNode
 {
+    public bool ShouldLogErrors => Value is not null;
+
     public override void BuildString(StringBuilder builder)
     {
         // Nothing to add.
@@ -12,7 +14,7 @@ internal class DataContextNode : ExpressionNode, ISourceNode
 
     public object SelectSource(object? source, object target, object? anchor)
     {
-        if (source is not null)
+        if (source != AvaloniaProperty.UnsetValue)
             throw new NotSupportedException(
                 "DataContextNode is invalid in conjunction with a binding source.");
         if (target is IDataContextProvider and AvaloniaObject)

src/Avalonia.Base/Data/Core/ExpressionNodes/ParentDataContextNode.cs

@@ -19,7 +19,7 @@ internal class ParentDataContextNode : ExpressionNode, ISourceNode
 
     public object SelectSource(object? source, object target, object? anchor)
     {
-        if (source is not null)
+        if (source != AvaloniaProperty.UnsetValue)
             throw new NotSupportedException(
                 "ParentDataContextNode is invalid in conjunction with a binding source.");
         if (target is IDataContextProvider and AvaloniaObject)

src/Avalonia.Base/Data/Core/ExpressionNodes/TemplatedParentNode.cs

@@ -12,7 +12,7 @@ internal class TemplatedParentNode : ExpressionNode, ISourceNode
 
     public object SelectSource(object? source, object target, object? anchor)
     {
-        if (source is not null)
+        if (source != AvaloniaProperty.UnsetValue)
             throw new NotSupportedException(
                 "TemplatedParentNode is invalid in conjunction with a binding source.");
         if (target is StyledElement)

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

@@ -50,14 +50,14 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
             // If the binding isn't rooted (i.e. doesn't have a Source or start with $parent, $self,
             // #elementName etc.) then we need to add a data context source node.
-            if (Source is null && !isRooted)
+            if (Source == AvaloniaProperty.UnsetValue && !isRooted)
                 nodes.Insert(0, ExpressionNodeFactory.CreateDataContext(targetProperty));
 
             // If the first node is an ISourceNode then allow it to select the source; otherwise
             // use the binding source if specified, falling back to the target.
             var source = nodes.Count > 0 && nodes[0] is ISourceNode sn
                 ? sn.SelectSource(Source, target, anchor ?? DefaultAnchor?.Target)
-                : Source ?? target;
+                : Source != AvaloniaProperty.UnsetValue? Source : target;
 
             // Create the binding expression and wrap it in an InstancedBinding.
             var expression = new BindingExpression(
@@ -87,7 +87,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
         /// </remarks>
         internal BindingExpression CreateObservableForTreeDataTemplate(object source)
         {
-            if (Source is not null)
+            if (Source != AvaloniaProperty.UnsetValue)
                 throw new NotSupportedException("Source bindings are not supported in this context.");
 
             var nodes = new List<ExpressionNode>();
@@ -109,7 +109,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
         [ConstructorArgument("path")]
         public CompiledBindingPath Path { get; set; }
 
-        public object? Source { get; set; }
+        public object? Source { get; set; } = AvaloniaProperty.UnsetValue;
 
         public Type? DataType { get; set; }
     }

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

@@ -54,7 +54,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
         public BindingPriority Priority { get; set; } = BindingPriority.LocalValue;
 
-        public object? Source { get; set; }
+        public object? Source { get; set; } = AvaloniaProperty.UnsetValue;
 
         public string? StringFormat { get; set; }
 

src/Markup/Avalonia.Markup/Data/Binding.cs

@@ -47,7 +47,7 @@ namespace Avalonia.Data
         /// <summary>
         /// Gets or sets the source for the binding.
         /// </summary>
-        public object? Source { get; set; }
+        public object? Source { get; set; } = AvaloniaProperty.UnsetValue;
 
         /// <summary>
         /// Gets or sets the binding path.
@@ -85,14 +85,14 @@ namespace Avalonia.Data
             // #elementName etc.) then we need to add a source node. The type of source node will
             // depend on the ElementName and RelativeSource properties of the binding and if
             // neither of those are set will default to a data context node.
-            if (Source is null && !isRooted &&  CreateSourceNode(targetProperty) is { } sourceNode)
+            if (Source == AvaloniaProperty.UnsetValue && !isRooted && CreateSourceNode(targetProperty) is { } sourceNode)
                 nodes.Insert(0, sourceNode);
 
             // If the first node is an ISourceNode then allow it to select the source; otherwise
             // use the binding source if specified, falling back to the target.
-            var source = nodes.Count > 0 && nodes[0] is ISourceNode sn
-                ? sn.SelectSource(Source, target, anchor ?? DefaultAnchor?.Target)
-                : Source ?? target;
+            var source = nodes.Count > 0 && nodes[0] is ISourceNode sn ?
+                sn.SelectSource(Source, target, anchor ?? DefaultAnchor?.Target) :
+                Source != AvaloniaProperty.UnsetValue ? Source : target;
 
             // Create the binding expression and wrap it in an InstancedBinding.
             var expression = new BindingExpression(
@@ -125,7 +125,7 @@ namespace Avalonia.Data
                 throw new NotSupportedException("ElementName bindings are not supported in this context.");
             if (RelativeSource is not null && RelativeSource.Mode != RelativeSourceMode.DataContext)
                 throw new NotSupportedException("RelativeSource bindings are not supported in this context.");
-            if (Source is not null)
+            if (Source != AvaloniaProperty.UnsetValue)
                 throw new NotSupportedException("Source bindings are not supported in this context.");
 
             var nodes = new List<ExpressionNode>();