Add support for constrained opcode (#299)

+semver:fix

Author: hazzik

src/DelegateDecompiler.Tests/Issue298.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Linq.Expressions;
+using NUnit.Framework;
+
+namespace DelegateDecompiler.Tests;
+
+[TestFixture]
+public class Issue298 : DecompilerTestsBase
+{
+    [Test]
+    public void TestNotConstrained()
+    {
+        static int NotConstrained<T>(T value) where T : ITestInterface => ((ITestInterface)value).Value;
+
+        Expression<Func<TestClass, int>> expected = value => value.Value;
+        Test(NotConstrained, expected);
+    }
+
+    [Test]
+    public void TestConstrained()
+    {
+        static int Constrained<T>(T value) where T : ITestInterface => value.Value;
+
+        Expression<Func<TestClass, int>> expected = value => value.Value;
+        Test(Constrained, expected);
+    }
+
+    interface ITestInterface
+    {
+        public int Value { get; }
+    }
+
+    class TestClass : ITestInterface
+    {
+        public int Value { get; set; }
+    }
+}

src/DelegateDecompiler/Processors/ConvertTypeProcessor.cs

@@ -10,13 +10,20 @@ internal class ConvertTypeProcessor : IProcessor
     {
         public static void Register(Dictionary<OpCode, IProcessor> processors)
         {
-            processors.Register(new ConvertTypeProcessor(), OpCodes.Castclass, OpCodes.Unbox, OpCodes.Unbox_Any);
+            processors.Register(new ConvertTypeProcessor(), 
+                OpCodes.Castclass, 
+                OpCodes.Unbox,
+                OpCodes.Unbox_Any,
+                OpCodes.Constrained);
         }
-    
+
         public void Process(ProcessorState state, Instruction instruction)
         {
             var expression = state.Stack.Pop();
-            state.Stack.Push(Expression.Convert(expression, (Type)instruction.Operand));
+            var targetType = (Type)instruction.Operand;
+            state.Stack.Push(expression.Type != targetType
+                ? Expression.Convert(expression, targetType)
+                : expression);
         }
     }
 }