Update CS0460 documentation to include C# 8/9 constraint exceptions

Co-authored-by: BillWagner <[email protected]>

Author: Copilot

docs/csharp/misc/cs0460.md

@@ -1,7 +1,7 @@
 ---
 description: "Compiler Error CS0460"
 title: "Compiler Error CS0460"
-ms.date: 07/20/2015
+ms.date: 01/02/2025
 f1_keywords: 
   - "CS0460"
 helpviewer_keywords: 
@@ -10,13 +10,17 @@ ms.assetid: 98d39ded-d3f9-4520-b912-892e574c056b
 ---
 # Compiler Error CS0460
 
-Constraints for override and explicit interface implementation methods are inherited from the base method, so they cannot be specified directly  
+Constraints for override and explicit interface implementation methods are inherited from the base method, so they cannot be specified directly, except in specific cases  
 
  When a generic method that is part of a derived class overrides a method in the base class, you may not specify constraints on the overridden method. The override method in the derived class inherits its constraints from the method in the base class.  
 
+ However, there are exceptions to this rule:  
+ - Starting with C# 9, you can apply the `default` constraint to override and explicit interface implementation methods to resolve ambiguities with nullable reference types.  
+ - Starting with C# 8, you can explicitly specify `where T : class` and `where T : struct` constraints on override and explicit interface implementation methods to allow annotations for type parameters constrained to reference types.  
+  
 ## Example  
 
- The following sample generates CS0460.  
+ The following sample generates CS0460 when attempting to redeclare inherited constraints.  
 
 ```csharp  
 // CS0460.cs  
@@ -30,12 +34,19 @@ interface I
 {  
    void F1<T>() where T : BaseClass;  
    void F2<T>() where T : struct;  
-   void F3<T>() where T : BaseClass;  
+   void F3<T>();  
+   void F4<T>() where T : struct;  
 }  
 
 class ExpImpl : I  
 {  
-   void I.F1<T>() where T : BaseClass {}   // CS0460  
-   void I.F2<T>() where T : class {}  // CS0460  
+   void I.F1<T>() where T : BaseClass {}   // CS0460 - cannot redeclare inherited constraint  
+   void I.F2<T>() where T : struct {}      // CS0460 - cannot redeclare inherited constraint  
+   
+   // Valid since C# 8 - explicit class constraint for nullable annotations  
+   void I.F4<T>() where T : class {}       // OK - explicit constraint for nullable annotations  
+   
+   // Valid since C# 9 - default constraint to resolve ambiguities  
+   void I.F3<T>() where T : default {}     // OK - default constraint  
 }  
 ```