Fix #3382: Support compiler-generated throw-helper invocations in switch-expression implicit default-case.

Author: siegfriedpammer

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

@@ -159,5 +159,59 @@ public static string Issue2222()
 				_ => "default",
 			};
 		}
+#pragma warning disable CS8509 // The switch expression does not handle all possible values of its input type (it is not exhaustive).
+		public static int Issue3382(StringComparison c)
+		{
+			return c switch {
+				StringComparison.Ordinal => 0,
+				StringComparison.OrdinalIgnoreCase => 1,
+			};
+		}
+
+		public static void Issue3382b(ref StringComparison? c)
+		{
+			StringComparison? stringComparison = c;
+			c = stringComparison switch {
+				null => StringComparison.Ordinal,
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+		}
+
+		public static void Issue3382c(StringComparison? c)
+		{
+			c = c switch {
+				null => StringComparison.Ordinal,
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+		}
+
+		public static void Issue3382d(ref StringComparison c)
+		{
+			StringComparison stringComparison = c;
+			c = stringComparison switch {
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+		}
+
+		public static int SwitchOnStringImplicitDefault(string s)
+		{
+			return s switch {
+				"Hello" => 42,
+				"World" => 4711,
+				"!" => 7,
+				"Foo" => 13,
+				"Bar" => 21,
+				"Baz" => 84,
+				"Qux" => 168,
+				"Quux" => 336,
+				"Corge" => 672,
+				"Grault" => 1344,
+				"Garply" => 2688,
+			};
+		}
+#pragma warning restore CS8509 // The switch expression does not handle all possible values of its input type (it is not exhaustive).
 	}
 }

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -4135,10 +4135,13 @@ protected internal override TranslatedExpression VisitSwitchInstruction(SwitchIn
 				switchExpr.SwitchSections.Add(ses);
 			}
 
-			var defaultSES = new SwitchExpressionSection();
-			defaultSES.Pattern = new IdentifierExpression("_");
-			defaultSES.Body = TranslateSectionBody(defaultSection);
-			switchExpr.SwitchSections.Add(defaultSES);
+			if (!defaultSection.IsCompilerGeneratedDefaultSection)
+			{
+				var defaultSES = new SwitchExpressionSection();
+				defaultSES.Pattern = new IdentifierExpression("_");
+				defaultSES.Body = TranslateSectionBody(defaultSection);
+				switchExpr.SwitchSections.Add(defaultSES);
+			}
 
 			return switchExpr.WithILInstruction(inst).WithRR(new ResolveResult(resultType));
 

ICSharpCode.Decompiler/IL/ControlFlow/SwitchDetection.cs

@@ -16,8 +16,10 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.FlowAnalysis;
@@ -216,12 +218,118 @@ void ProcessBlock(Block block, ref bool blockContainerNeedsCleanup)
 				// (1st pass was in ControlFlowSimplification)
 				SimplifySwitchInstruction(block, context);
 			}
+
+			InlineSwitchExpressionDefaultCaseThrowHelper(block, context);
+		}
+
+		// call ThrowInvalidOperationException(...)
+		// leave IL_0000 (ldloc temp)
+		//
+		// -or-
+		//
+		// call ThrowSwitchExpressionException(...)
+		// leave IL_0000 (ldloc temp)
+		// 
+		// -to-
+		//
+		// throw(newobj SwitchExpressionException(...))
+		internal static void InlineSwitchExpressionDefaultCaseThrowHelper(Block block, ILTransformContext context)
+		{
+#nullable enable
+			// due to our of of basic blocks at this point,
+			// switch instructions can only appear as last instruction
+			var sw = block.Instructions.LastOrDefault() as SwitchInstruction;
+			if (sw == null)
+				return;
+
+			IMethod[] exceptionCtorTable = new IMethod[2];
+
+			exceptionCtorTable[0] = FindConstructor("System.InvalidOperationException")!;
+			exceptionCtorTable[1] = FindConstructor("System.Runtime.CompilerServices.SwitchExpressionException", typeof(object))!;
+
+			if (exceptionCtorTable[0] == null && exceptionCtorTable[1] == null)
+				return;
+
+			if (sw.GetDefaultSection() is not { Body: Branch { TargetBlock: Block defaultBlock } } defaultSection)
+				return;
+			if (defaultBlock is { Instructions: [var call, Branch or Leave] })
+			{
+				if (!MatchThrowHelperCall(call, out IMethod? exceptionCtor, out ILInstruction? value))
+					return;
+				context.Step("SwitchExpressionDefaultCaseTransform", block.Instructions[0]);
+				var newObj = new NewObj(exceptionCtor);
+				if (value != null)
+					newObj.Arguments.Add(value);
+				defaultBlock.Instructions[0] = new Throw(newObj).WithILRange(defaultBlock.Instructions[0]).WithILRange(defaultBlock.Instructions[1]);
+				defaultBlock.Instructions.RemoveAt(1);
+				defaultSection.IsCompilerGeneratedDefaultSection = true;
+			}
+			else if (defaultBlock is { Instructions: [Throw { Argument: NewObj { Method: var ctor, Arguments: [var arg] } newObj }] }
+				&& ctor.Equals(exceptionCtorTable[1]))
+			{
+				defaultSection.IsCompilerGeneratedDefaultSection = true;
+			}
+			else
+			{
+				return;
+			}
+
+			bool MatchThrowHelperCall(ILInstruction inst, [NotNullWhen(true)] out IMethod? exceptionCtor, out ILInstruction? value)
+			{
+				exceptionCtor = null;
+				value = null;
+				if (inst is not Call call)
+					return false;
+				if (call.Method.DeclaringType.FullName != "<PrivateImplementationDetails>")
+					return false;
+				switch (call.Arguments.Count)
+				{
+					case 0:
+						if (call.Method.Name != "ThrowInvalidOperationException")
+							return false;
+						exceptionCtor = exceptionCtorTable[0];
+						break;
+					case 1:
+						if (call.Method.Name != "ThrowSwitchExpressionException")
+							return false;
+						exceptionCtor = exceptionCtorTable[1];
+						value = call.Arguments[0];
+						break;
+					default:
+						return false;
+				}
+				return exceptionCtor != null;
+			}
+
+			IMethod? FindConstructor(string fullTypeName, params Type[] argumentTypes)
+			{
+				IType exceptionType = context.TypeSystem.FindType(new FullTypeName(fullTypeName));
+				var types = argumentTypes.SelectArray(context.TypeSystem.FindType);
+
+				foreach (var ctor in exceptionType.GetConstructors(m => !m.IsStatic && m.Parameters.Count == argumentTypes.Length))
+				{
+					bool found = true;
+					foreach (var pair in ctor.Parameters.Select(p => p.Type).Zip(types))
+					{
+						if (!NormalizeTypeVisitor.IgnoreNullability.EquivalentTypes(pair.Item1, pair.Item2))
+						{
+							found = false;
+							break;
+						}
+					}
+					if (found)
+						return ctor;
+				}
+
+				return null;
+			}
+#nullable restore
 		}
 
 		internal static void SimplifySwitchInstruction(Block block, ILTransformContext context)
 		{
 			// due to our of of basic blocks at this point,
-			// switch instructions can only appear as last insturction
+			// switch instructions can only appear as last instruction
 			var sw = block.Instructions.LastOrDefault() as SwitchInstruction;
 			if (sw == null)
 				return;
@@ -445,7 +553,9 @@ private void AddNullCase(List<ControlFlowNode> flowNodes, List<ControlFlowNode>
 				!nullableBlock.Instructions.SecondToLastOrDefault().MatchIfInstruction(out var cond, out var trueInst) ||
 				!cond.MatchLogicNot(out var getHasValue) ||
 				!NullableLiftingTransform.MatchHasValueCall(getHasValue, out ILInstruction nullableInst))
+			{
 				return;
+			}
 
 			// could check that nullableInst is ldloc or ldloca and that the switch variable matches a GetValueOrDefault
 			// but the effect of adding an incorrect block to the flowBlock list would only be disasterous if it branched directly

ICSharpCode.Decompiler/IL/Instructions/SwitchInstruction.cs

@@ -202,6 +202,12 @@ public SwitchSection()
 		/// </summary>
 		public bool HasNullLabel { get; set; }
 
+		/// <summary>
+		/// If true, this section only contains a compiler-generated throw helper
+		/// used in a switch expression and will not be visible in the decompiled source code.
+		/// </summary>
+		public bool IsCompilerGeneratedDefaultSection { get; set; }
+
 		/// <summary>
 		/// The set of labels that cause execution to jump to this switch section.
 		/// </summary>
@@ -221,6 +227,8 @@ public override InstructionFlags DirectFlags {
 		public override void WriteTo(ITextOutput output, ILAstWritingOptions options)
 		{
 			WriteILRange(output, options);
+			if (IsCompilerGeneratedDefaultSection)
+				output.Write("generated.");
 			output.WriteLocalReference("case", this, isDefinition: true);
 			output.Write(' ');
 			if (HasNullLabel)

ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs

@@ -1169,7 +1169,11 @@ SwitchInstruction ReplaceWithSwitchInstruction(int offset)
 				}
 				var newSwitch = new SwitchInstruction(new StringToInt(switchValueInst, values, switchValueLoad.Variable.Type));
 				newSwitch.Sections.AddRange(sections);
-				newSwitch.Sections.Add(new SwitchSection { Labels = defaultLabel, Body = defaultSection.Body });
+				newSwitch.Sections.Add(new SwitchSection {
+					Labels = defaultLabel,
+					Body = defaultSection.Body,
+					IsCompilerGeneratedDefaultSection = defaultSection.IsCompilerGeneratedDefaultSection
+				});
 				instructions[offset].ReplaceWith(newSwitch);
 				return newSwitch;
 			}
@@ -1310,6 +1314,7 @@ private bool MatchRoslynSwitchOnStringUsingLengthAndChar(Block block, int i)
 			}
 			instructions[i] = newSwitch;
 			instructions.RemoveRange(i + 1, instructions.Count - (i + 1));
+			SwitchDetection.InlineSwitchExpressionDefaultCaseThrowHelper(block, context);
 			return true;
 
 			bool MatchGetChars(ILInstruction instruction, ILVariable switchValueVar, out int index)