feat: Extend Visitor generator to support interface and struct hierarchies

docs/generators/visitor-generator.md

@@ -1,6 +1,6 @@
 # Visitor Pattern Generator
 
-The Visitor Pattern Generator automatically generates fluent, type-safe visitor infrastructure for class hierarchies marked with the `[GenerateVisitor]` attribute. This eliminates boilerplate code and provides modern C# ergonomics including async/await support, ValueTask, and generic type inference.
+The Visitor Pattern Generator automatically generates fluent, type-safe visitor infrastructure for hierarchies marked with the `[GenerateVisitor]` attribute. It supports class, interface, struct, and record hierarchies, eliminating boilerplate code and providing modern C# ergonomics including async/await support, ValueTask, and generic type inference.
 
 ## Overview
 
@@ -118,6 +118,129 @@ var asyncLogger = new AstNodeAsyncActionVisitorBuilder()
 await myExpression.AcceptAsync(asyncLogger);
 ```
 
+## Supported Hierarchy Types
+
+The visitor generator supports multiple types of hierarchies, providing flexibility in design:
+
+### Class-Based Hierarchies
+
+Traditional class inheritance hierarchies are fully supported:
+
+```csharp
+[GenerateVisitor]
+public abstract partial class Animal
+{
+}
+
+public partial class Dog : Animal
+{
+    public string Breed { get; init; }
+}
+
+public partial class Cat : Animal
+{
+    public bool IsIndoor { get; init; }
+}
+```
+
+### Interface-Based Hierarchies
+
+Hierarchies based on interfaces work seamlessly:
+
+```csharp
+[GenerateVisitor]
+public partial interface IShape
+{
+}
+
+public partial class Circle : IShape
+{
+    public double Radius { get; init; }
+}
+
+public partial class Rectangle : IShape
+{
+    public double Width { get; init; }
+    public double Height { get; init; }
+}
+
+public partial class Triangle : IShape
+{
+    public double Base { get; init; }
+    public double Height { get; init; }
+}
+```
+
+**Note:** For interface base types, the generated visitor interface name is intelligently derived. `IShape` generates `IShapeVisitor` (not `IIShapeVisitor`).
+
+### Struct-Based Hierarchies
+
+Value types can implement visitable interfaces for allocation-free visitor patterns:
+
+```csharp
+[GenerateVisitor]
+public partial interface IValue
+{
+}
+
+public partial struct IntValue : IValue
+{
+    public int Value { get; init; }
+}
+
+public partial struct DoubleValue : IValue
+{
+    public double Value { get; init; }
+}
+
+// No boxing occurs during visitation
+var visitor = new IValueVisitorBuilder<string>()
+    .When<IntValue>(i => $"Int:{i.Value}")
+    .When<DoubleValue>(d => $"Double:{d.Value:F2}")
+    .Build();
+
+var intVal = new IntValue { Value = 42 };
+var result = intVal.Accept(visitor); // "Int:42"
+```
+
+### Record Types
+
+Records are also supported:
+
+```csharp
+[GenerateVisitor]
+public abstract partial record Message;
+
+public partial record TextMessage(string Content) : Message;
+public partial record ImageMessage(byte[] Data, string Format) : Message;
+```
+
+### Mixed Hierarchies
+
+You can mix interfaces, classes, and structs in complex hierarchies:
+
+```csharp
+[GenerateVisitor]
+public partial interface INode
+{
+}
+
+public abstract partial class Expression : INode
+{
+}
+
+public partial class Literal : Expression
+{
+    public object Value { get; init; }
+}
+
+public partial struct Position : INode
+{
+    public int Line { get; init; }
+    public int Column { get; init; }
+}
+```
+
 ## Attribute Options
 
 The `[GenerateVisitor]` attribute supports several options:
@@ -391,6 +514,62 @@ var validator = new DocumentVisitorBuilder<ValidationResult>()
     .Build();
 ```
 
+## Diagnostics
+
+The generator provides helpful diagnostics to catch common issues:
+
+### PKVIS001: No concrete types found
+
+**Severity:** Warning
+
+This warning appears when the generator cannot find any concrete types implementing or deriving from the marked base type.
+
+```csharp
+[GenerateVisitor]
+public partial interface IEmptyHierarchy { }
+
+// Warning PKVIS001: No concrete types implementing or deriving from 'IEmptyHierarchy' were found
+```
+
+**Solutions:**
+- Add concrete types that implement the interface or derive from the class
+- Set `AutoDiscoverDerivedTypes = false` if you're building types manually
+
+### PKVIS002: Type must be partial
+
+**Severity:** Error
+
+The base type (class, struct, or interface) must be declared as `partial` to allow Accept method generation.
+
+```csharp
+[GenerateVisitor]
+public class NonPartialBase { } // Error!
+
+// Fix:
+[GenerateVisitor]
+public partial class PartialBase { } // Correct
+```
+
+**Solution:** Add the `partial` keyword to the type declaration.
+
+### PKVIS004: Derived type must be partial
+
+**Severity:** Error
+
+All derived types must be `partial` to allow Accept method generation.
+
+```csharp
+[GenerateVisitor]
+public partial class Base { }
+
+public class Derived : Base { } // Error!
+
+// Fix:
+public partial class Derived : Base { } // Correct
+```
+
+**Solution:** Add the `partial` keyword to all derived types in the hierarchy.
+
 ## Troubleshooting
 
 ### "No handler registered for type X"

src/PatternKit.Generators.Abstractions/Visitors/VisitorAttributes.cs

@@ -12,16 +12,25 @@ namespace PatternKit.Generators.Visitors;
 /// <item>Fluent builder APIs for composing visitors</item>
 /// </list>
 /// <example>
+/// <para>Class-based hierarchy:</para>
 /// <code>
 /// [GenerateVisitor]
 /// public partial class AstNode { }
 /// 
 /// public partial class Expression : AstNode { }
 /// public partial class Statement : AstNode { }
 /// </code>
+/// <para>Interface-based hierarchy:</para>
+/// <code>
+/// [GenerateVisitor]
+/// public partial interface IShape { }
+/// 
+/// public partial class Circle : IShape { }
+/// public partial class Rectangle : IShape { }
+/// </code>
 /// </example>
 /// </remarks>
-[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
 public sealed class GenerateVisitorAttribute : Attribute
 {
     /// <summary>

src/PatternKit.Generators/AnalyzerReleases.Unshipped.md

@@ -37,3 +37,6 @@ PKMEM003 | PatternKit.Generators.Memento | Warning | Unsafe reference capture
 PKMEM004 | PatternKit.Generators.Memento | Error | Clone strategy requested but mechanism missing
 PKMEM005 | PatternKit.Generators.Memento | Error | Record restore generation failed
 PKMEM006 | PatternKit.Generators.Memento | Info | Init-only or readonly restrictions prevent in-place restore
+PKVIS001 | PatternKit.Generators.Visitor | Warning | No concrete types found for visitor generation
+PKVIS002 | PatternKit.Generators.Visitor | Error | Type must be partial for Accept method generation
+PKVIS004 | PatternKit.Generators.Visitor | Error | Derived type must be partial for Accept method generation