Fix #1621: Add AllowUnreachableCases in switch analysis.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.cs

@@ -1145,5 +1145,28 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 			Console.WriteLine();
 		}
+
+		public static string Issue1621(int x)
+		{
+			if (x == 5) {
+				return "5";
+			}
+			switch (x) {
+				case 1:
+					return "1";
+				case 2:
+				case 6:
+				case 7:
+					return "2-6-7";
+				case 3:
+					return "3";
+				case 4:
+					return "4";
+				case 5:
+					return "unreachable";
+				default:
+					throw new Exception();
+			}
+		}
 	}
 }

ICSharpCode.Decompiler/IL/ControlFlow/SwitchAnalysis.cs

@@ -28,10 +28,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		/// <summary>
 		/// The variable to be used as the argument of the switch instruction.
 		/// </summary>
-		public ILVariable SwitchVariable
-		{
-			get { return switchVar; }
-		}
+		public ILVariable SwitchVariable => switchVar;
 
 		/// <summary>
 		/// Whether at least one the analyzed blocks contained an IL switch constructors.
@@ -62,6 +59,11 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		
 		public Block RootBlock { get; private set; }
 
+		/// <summary>
+		/// Gets/sets whether to allow unreachable cases in switch instructions.
+		/// </summary>
+		public bool AllowUnreachableCases { get; set; }
+
 		/// <summary>
 		/// Analyze the last two statements in the block and see if they can be turned into a
 		/// switch instruction.
@@ -180,6 +182,8 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			}
 			foreach (var section in inst.Sections) {
 				var matchValues = section.Labels.AddOffset(offset).IntersectWith(inputValues);
+				if (!AllowUnreachableCases && matchValues.IsEmpty)
+					return false;
 				if (matchValues.Count() > 1 && section.Body.MatchBranch(out var targetBlock) && AnalyzeBlock(targetBlock, matchValues)) {
 					InnerBlocks.Add(targetBlock);
 				} else {

ICSharpCode.Decompiler/IL/ControlFlow/SwitchDetection.cs

@@ -132,6 +132,8 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		{
 			this.context = context;
 
+			analysis.AllowUnreachableCases = context.Settings.RemoveDeadCode;
+
 			foreach (var container in function.Descendants.OfType<BlockContainer>()) {
 				currentContainer = container;
 				controlFlowGraph = null;