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

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -141,6 +141,20 @@ namespace ICSharpCode.Decompiler.Tests
 			CompilerOptions.Optimize | CompilerOptions.UseRoslynLatest,
 		});
 
+		static readonly CompilerOptions[] roslyn3OrNewerWithNet40Roslyn4Options = Tester.SupportedOnCurrentPlatform(new[]
+		{
+			CompilerOptions.UseRoslyn4_14_0 | CompilerOptions.TargetNet40,
+			CompilerOptions.Optimize | CompilerOptions.UseRoslyn4_14_0 | CompilerOptions.TargetNet40,
+			CompilerOptions.UseRoslynLatest | CompilerOptions.TargetNet40,
+			CompilerOptions.Optimize | CompilerOptions.UseRoslynLatest | CompilerOptions.TargetNet40,
+			CompilerOptions.UseRoslyn3_11_0,
+			CompilerOptions.Optimize | CompilerOptions.UseRoslyn3_11_0,
+			CompilerOptions.UseRoslyn4_14_0,
+			CompilerOptions.Optimize | CompilerOptions.UseRoslyn4_14_0,
+			CompilerOptions.UseRoslynLatest,
+			CompilerOptions.Optimize | CompilerOptions.UseRoslynLatest,
+		});
+
 		static readonly CompilerOptions[] roslyn3OrNewerOptions = Tester.SupportedOnCurrentPlatform(new[]
 		{
 			CompilerOptions.UseRoslyn3_11_0,
@@ -291,8 +305,14 @@ namespace ICSharpCode.Decompiler.Tests
 			});
 		}
 
+		// Runs on Roslyn 3.x and Roslyn 4.x or newer, with TargetNet40 variants only for
+		// Roslyn 4.x or newer: targeting net40 makes the compiler emit the
+		// ThrowInvalidOperationException throw helper (SwitchExpressionException does not
+		// exist there), but Roslyn 3.x emits a plain inline
+		// "throw new InvalidOperationException()" instead, which is indistinguishable from
+		// user code and therefore intentionally not transformed.
 		[Test]
-		public async Task SwitchExpressions([ValueSource(nameof(roslyn3OrNewerOptions))] CompilerOptions cscOptions)
+		public async Task SwitchExpressions([ValueSource(nameof(roslyn3OrNewerWithNet40Roslyn4Options))] CompilerOptions cscOptions)
 		{
 			await RunForLibrary(cscOptions: cscOptions);
 		}

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

@@ -159,5 +159,175 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				_ => "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)
+		{
+#if NET40
+			c = c switch {
+				null => StringComparison.Ordinal,
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+#else
+			StringComparison? stringComparison = c;
+			c = stringComparison switch {
+				null => StringComparison.Ordinal,
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+#endif
+		}
+
+		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)
+		{
+#if NET40
+			c = c switch {
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+#else
+			StringComparison stringComparison = c;
+			c = stringComparison switch {
+				StringComparison.Ordinal => StringComparison.OrdinalIgnoreCase,
+				StringComparison.OrdinalIgnoreCase => StringComparison.InvariantCulture,
+			};
+#endif
+		}
+
+		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,
+			};
+		}
+		public static int SwitchOnStringImplicitDefaultFewCases(string s)
+		{
+			return s switch {
+				"red" => 1,
+				"green" => 2,
+				"blue" => 3,
+			};
+		}
+
+		public static int SwitchOnStringImplicitDefaultUniqueLengths(string s)
+		{
+			return s switch {
+				"a" => 1,
+				"bb" => 2,
+				"ccc" => 3,
+				"dddd" => 4,
+				"eeeee" => 5,
+				"ffffff" => 6,
+				"ggggggg" => 7,
+				"hhhhhhhh" => 8,
+				"iiiiiiiii" => 9,
+			};
+		}
+
+		public static int SwitchOnStringImplicitDefaultSameLength(string s)
+		{
+			return s switch {
+				"aabb" => 1,
+				"abab" => 2,
+				"abba" => 3,
+				"baab" => 4,
+				"baba" => 5,
+				"bbaa" => 6,
+				"bbbb" => 7,
+				"aaab" => 8,
+				"aaba" => 9,
+			};
+		}
+
+		public static int SwitchOnStringImplicitDefaultWithNullCase(string s)
+		{
+			return s switch {
+				"a" => 1,
+				"bb" => 2,
+				"ccc" => 3,
+				"dddd" => 4,
+				"eeeee" => 5,
+				"ffffff" => 6,
+				"ggggggg" => 7,
+				"hhhhhhhh" => 8,
+				"iiiiiiiii" => 9,
+				null => -1,
+			};
+		}
+
+		public static int SwitchOnStringImplicitDefaultTwoCases(string s)
+		{
+			return s switch {
+				"red" => 1,
+				"green" => 2,
+			};
+		}
+
+		public static string SwitchOnCharImplicitDefault(char c)
+		{
+			return c switch {
+				'a' => "first",
+				'b' => "second",
+				'c' => "third",
+			};
+		}
+
+		public static string SwitchOnLongImplicitDefault(long l)
+		{
+			return l switch {
+				1L => "one",
+				100L => "hundred",
+				10000L => "ten thousand",
+				long.MaxValue => "max",
+			};
+		}
+
+		public static void SwitchExpressionAsArgumentImplicitDefault(int i)
+		{
+			Console.WriteLine(i switch {
+				0 => "zero",
+				5 => "five",
+				10 => "ten",
+			});
+		}
+
+		public static int SwitchExpressionNestedImplicitDefault(StringComparison c, int i)
+		{
+			return c switch {
+				StringComparison.Ordinal => i switch {
+					0 => 1,
+					1 => 2,
+				},
+				StringComparison.OrdinalIgnoreCase => 3,
+			};
+		}
+#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 @@ namespace ICSharpCode.Decompiler.CSharp
 				switchExpr.SwitchSections.Add(ses);
 			}
 
-			var defaultSES = new SwitchExpressionSection();
+			if (!defaultSection.IsCompilerGeneratedDefaultSection)
-			defaultSES.Pattern = new IdentifierExpression("_");
+			{
-			defaultSES.Body = TranslateSectionBody(defaultSection);
+				var defaultSES = new SwitchExpressionSection();
-			switchExpr.SwitchSections.Add(defaultSES);
+				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 +218,150 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				// (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 use 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: [_] } }] }
+				&& ctor.Equals(exceptionCtorTable[1]))
+			{
+				defaultSection.IsCompilerGeneratedDefaultSection = true;
+			}
+			else
+			{
+				return;
+			}
+
+			bool MatchThrowHelperCall(ILInstruction inst, [NotNullWhen(true)] out IMethod? exceptionCtor, out ILInstruction? value)
+			{
+				exceptionCtor = null;
+				if (!MatchSwitchExpressionThrowHelperCall(inst, out value))
+					return false;
+				exceptionCtor = value == null ? exceptionCtorTable[0] : exceptionCtorTable[1];
+				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
+		}
+
+#nullable enable
+		/// <summary>
+		/// Matches a call to one of the compiler-generated throw helpers in
+		/// &lt;PrivateImplementationDetails&gt; that implement the implicit default case of a
+		/// switch expression. <paramref name="value"/> is the switch value passed to
+		/// ThrowSwitchExpressionException, or null for the parameterless
+		/// ThrowInvalidOperationException helper (emitted when SwitchExpressionException
+		/// is not available on the target framework).
+		/// </summary>
+		static bool MatchSwitchExpressionThrowHelperCall(ILInstruction inst, out ILInstruction? value)
+		{
+			value = null;
+			if (inst is not Call call)
+				return false;
+			if (call.Method.DeclaringType.FullName != "<PrivateImplementationDetails>")
+				return false;
+			switch (call.Arguments.Count)
+			{
+				case 0:
+					return call.Method.Name == "ThrowInvalidOperationException";
+				case 1:
+					if (call.Method.Name != "ThrowSwitchExpressionException")
+						return false;
+					value = call.Arguments[0];
+					return true;
+				default:
+					return false;
+			}
+		}
+
+		/// <summary>
+		/// Returns true if the block consists solely of the compiler-generated default case
+		/// of a switch expression: either a throw-helper call followed by the unreachable
+		/// branch/leave to the end of the expression, or a directly inlined
+		/// "throw new SwitchExpressionException(...)" (emitted by older Roslyn versions).
+		/// </summary>
+		internal static bool IsSwitchExpressionThrowHelperBlock(Block block)
+		{
+			switch (block.Instructions)
+			{
+				case [var call, Branch or Leave]:
+					return MatchSwitchExpressionThrowHelperCall(call, out _);
+				case [Throw { Argument: NewObj { Method.DeclaringType: var exceptionType } }]:
+					return exceptionType.FullName == "System.Runtime.CompilerServices.SwitchExpressionException";
+				default:
+					return false;
+			}
 		}
+#nullable restore
 
 		internal static void SimplifySwitchInstruction(Block block, ILTransformContext context)
 		{
-			// due to our of of basic blocks at this point,
+			// due to our use 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;
@@ -452,7 +592,9 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				!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 @@ namespace ICSharpCode.Decompiler.IL
 		/// </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 @@ namespace ICSharpCode.Decompiler.IL
 		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

@@ -88,6 +88,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				if (!changed)
 					continue;
 				SwitchDetection.SimplifySwitchInstruction(block, context);
+				SwitchDetection.InlineSwitchExpressionDefaultCaseThrowHelper(block, context);
 				if (block.Parent is BlockContainer container)
 					changedContainers.Add(container);
 			}
@@ -330,9 +331,14 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				currentCaseBlock = nextCaseBlock as Block;
 			} while (currentCaseBlock != null);
 
-			// We didn't find enough cases, exit
+			// Short if-chains are more plausibly hand-written than a switch, so we require
-			if (values.Count < 3)
+			// at least 3 cases - unless the chain ends in a compiler-generated throw helper,
+			// which proves that the source was a switch expression with an implicit default case.
+			if (values.Count < 3
+				&& !(currentCaseBlock != null && SwitchDetection.IsSwitchExpressionThrowHelperBlock(currentCaseBlock)))
+			{
 				return false;
+			}
 			context.Step(nameof(SimplifyCascadingIfStatements), instructions[i]);
 			// if the switchValueVar is used in other places as well, do not eliminate the store.
 			if (switchValueVar.LoadCount > numberOfUniqueMatchesWithCurrentVariable || !ValidateUsesOfSwitchValueVariable(switchValueVar, caseBlocks))
@@ -1169,7 +1175,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				}
 				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;
 			}
@@ -1217,6 +1227,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				switchOnLengthBlockStartOffset = i;
 			}
 			ILInstruction defaultCase = null;
+			bool defaultIsCompilerGenerated = false;
 			if (!MatchSwitchOnLengthBlock(ref switchValueVar, switchOnLengthBlock, switchOnLengthBlockStartOffset, out var blocksByLength))
 				return false;
 			List<(string, ILInstruction)> stringValues = new();
@@ -1302,7 +1313,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 			var newSwitch = new SwitchInstruction(new StringToInt(new LdLoc(switchValueVar), values, switchValueVar.Type));
 			newSwitch.Sections.AddRange(sections);
-			newSwitch.Sections.Add(new SwitchSection { Labels = defaultLabel, Body = defaultCase is Block b2 ? new Branch(b2) : defaultCase });
+			newSwitch.Sections.Add(new SwitchSection {
+				Labels = defaultLabel,
+				Body = defaultCase is Block b2 ? new Branch(b2) : defaultCase,
+				IsCompilerGeneratedDefaultSection = defaultIsCompilerGenerated
+			});
 			newSwitch.AddILRange(instructions[i]);
 			if (nullCase != null)
 			{
@@ -1312,6 +1327,15 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			instructions.RemoveRange(i + 1, instructions.Count - (i + 1));
 			return true;
 
+			void InheritCompilerGeneratedDefaultMarker(SwitchInstruction inner)
+			{
+				// The char switches and the switch on the string length share the throw-helper
+				// block of the implicit default case; whichever of them was processed first by
+				// SwitchDetection carries the compiler-generated marker for it.
+				defaultIsCompilerGenerated |= inner.Sections.Any(s => s.IsCompilerGeneratedDefaultSection
+					&& s.Body.MatchBranch(out var target) && target == defaultCase);
+			}
+
 			bool MatchGetChars(ILInstruction instruction, ILVariable switchValueVar, out int index)
 			{
 				index = -1;
@@ -1350,6 +1374,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 							return false;
 						if (!MatchGetChars(@switch.Value, switchValueVar, out index))
 							return false;
+						InheritCompilerGeneratedDefaultMarker(@switch);
 						sections = @switch.Sections.SelectList(s => new KeyValuePair<LongSet, ILInstruction>(s.Labels, s.Body));
 						break;
 					case 2:
@@ -1364,6 +1389,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 							return false;
 						if (!@switch.Value.MatchLdLoc(charTempVar))
 							return false;
+						InheritCompilerGeneratedDefaultMarker(@switch);
 						sections = @switch.Sections.SelectList(s => new KeyValuePair<LongSet, ILInstruction>(s.Labels, s.Body));
 						break;
 					default:
@@ -1514,6 +1540,10 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						defaultCase ??= targetInst;
 						if (defaultCase != targetInst)
 							return false;
+						// The default section of the switch on the string length carries the
+						// compiler-generated marker if an implicit-default throw helper was
+						// already inlined there; transfer it to the rebuilt switch.
+						defaultIsCompilerGenerated |= section.IsCompilerGeneratedDefaultSection;
 					}
 					else
 					{