Fix #1936: TransformDisplayClassUsage should remove copies of display-class references.

ICSharpCode.Decompiler.Tests/TestCases/Correctness/Capturing.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 {
@@ -18,6 +19,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			InsideLoopOverArray2();
 			NotWhileDueToVariableInsideLoop();
 			NotDoWhileDueToVariableInsideLoop();
+			Issue1936();
 		}
 
 		static void TestCase1()
@@ -210,5 +212,23 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 				Console.WriteLine(f());
 			}
 		}
+
+		public static void Issue1936()
+		{
+			IEnumerable<object> outerCapture = null;
+			for (int i = 0; i < 10; i++) {
+				int innerCapture = 0;
+				Action a = (delegate {
+					List<object> list = new List<object>();
+					Console.WriteLine("before inc: " + innerCapture);
+					++innerCapture;
+					Console.WriteLine("after inc: " + innerCapture);
+					Console.WriteLine("before assign: " + outerCapture);
+					outerCapture = outerCapture == null ? list : outerCapture.Concat(list);
+					Console.WriteLine("after assign: " + outerCapture);
+				});
+				a.Invoke();
+			}
+		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -309,9 +309,18 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			base.VisitStLoc(inst);
 
-			if (inst.Variable.Kind == VariableKind.Local && inst.Variable.IsSingleDefinition && inst.Variable.LoadCount == 0 && inst.Value is StLoc) {
-				context.Step($"Remove unused variable assignment {inst.Variable.Name}", inst);
-				inst.ReplaceWith(inst.Value);
+			if (inst.Parent is Block && inst.Variable.IsSingleDefinition) {
+				if (inst.Variable.Kind == VariableKind.Local && inst.Variable.LoadCount == 0 && inst.Value is StLoc) {
+					context.Step($"Remove unused variable assignment {inst.Variable.Name}", inst);
+					inst.ReplaceWith(inst.Value);
+					return;
+				}
+				if (inst.Value.MatchLdLoc(out var displayClassVariable) && displayClasses.TryGetValue(displayClassVariable, out var displayClass)) {
+					context.Step($"Found copy-assignment of display-class variable {displayClassVariable.Name}", inst);
+					displayClasses.Add(inst.Variable, displayClass);
+					instructionsToRemove.Add(inst);
+					return;
+				}
 			}
 		}