Fix an edge case with XAML enum types

Author: Sergio0694

components/DependencyPropertyGenerator/CommunityToolkit.DependencyPropertyGenerator.SourceGenerators/Diagnostics/Analyzers/UseGeneratedDependencyPropertyOnManualPropertyAnalyzer.cs

@@ -473,11 +473,12 @@ void HandleSetAccessor(IPropertySymbol propertySymbol, PropertyFlags propertyFla
                                 // We have found a valid constant. If it's an enum type, we have a couple special cases to handle.
                                 if (conversionOperation.Operand.Type is { TypeKind: TypeKind.Enum } operandType)
                                 {
-                                    // As an optimization, we check whether the constant was the value
+                                    // As an optimization, we check whether the constant was the default value (not other enum member)
                                     // of some projected built-in WinRT enum type (ie. not any user-defined enum type). If that is the
                                     // case, the XAML infrastructure can default that values automatically, meaning we can skip the
                                     // overhead of instantiating a 'PropertyMetadata' instance in code, and marshalling default value.
-                                    if (operandType.IsContainedInNamespace(WellKnownTypeNames.XamlNamespace(useWindowsUIXaml)))
+                                    // We also need to exclude scenarios where the property is actually nullable, but we're assigning a value.
+                                    if (operandType.IsContainedInNamespace(WellKnownTypeNames.XamlNamespace(useWindowsUIXaml)) && !isNullableValueType)
                                     {
                                         // Before actually enabling the optimization, validate that the default value is actually
                                         // the same as the default value of the enum (ie. the value of its first declared field).

components/DependencyPropertyGenerator/CommunityToolkit.DependencyPropertyGenerator.Tests/Test_UseGeneratedDependencyPropertyOnManualPropertyCodeFixer.cs

@@ -2443,8 +2443,8 @@ public abstract partial class KeyFrame<TValue, TKeyFrame> : DependencyObject
     [DataRow("Visibility", "object", "new PropertyMetadata(default(Visibility))", "(PropertyType = typeof(object))")]
     [DataRow("Visibility?", "Visibility?", "null", "")]
     [DataRow("Visibility?", "Visibility?", "new PropertyMetadata(null)", "")]
-    //[DataRow("Visibility?", "Visibility?", "new PropertyMetadata(default(Visibility))", "(DefaultValue = Visibility.Visible)")]
-    //[DataRow("Visibility?", "Visibility?", "new PropertyMetadata(Visibility.Visible)", "(DefaultValue = Visibility.Visible)")]
+    [DataRow("Visibility?", "Visibility?", "new PropertyMetadata(default(Visibility))", "(DefaultValue = Visibility.Visible)")]
+    [DataRow("Visibility?", "Visibility?", "new PropertyMetadata(Visibility.Visible)", "(DefaultValue = Visibility.Visible)")]
     [DataRow("Visibility?", "Visibility?", "new PropertyMetadata(Visibility.Collapsed)", "(DefaultValue = Visibility.Collapsed)")]
     [DataRow("Visibility?", "object", "null", "(PropertyType = typeof(object))")]
     [DataRow("Visibility?", "object", "new PropertyMetadata(null)", "(PropertyType = typeof(object))")]
@@ -2459,7 +2459,7 @@ public abstract partial class KeyFrame<TValue, TKeyFrame> : DependencyObject
     [DataRow("MyEnum", "object", "new PropertyMetadata(default(MyEnum))", "(PropertyType = typeof(object))")]
     [DataRow("MyEnum?", "MyEnum?", "null", "")]
     [DataRow("MyEnum?", "MyEnum?", "new PropertyMetadata(null)", "")]
-    //[DataRow("MyEnum?", "MyEnum?", "new PropertyMetadata(default(MyEnum))", "(DefaultValue = MyEnum.A)")]
+    [DataRow("MyEnum?", "MyEnum?", "new PropertyMetadata(default(MyEnum))", "(DefaultValue = MyEnum.A)")]
     [DataRow("MyEnum?", "MyEnum?", "new PropertyMetadata(MyEnum.A)", "(DefaultValue = MyEnum.A)")]
     [DataRow("MyEnum?", "MyEnum?", "new PropertyMetadata(MyEnum.B)", "(DefaultValue = MyEnum.B)")]
     [DataRow("MyEnum?", "object", "null", "(PropertyType = typeof(object))")]