Add support for simple constant patterns.

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

@@ -310,6 +310,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+#if CS80
 		public void RecursivePattern_Type(object x)
 		{
 			if (x is X { C: string text })
@@ -322,6 +323,19 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		public void RecursivePattern_Constant(object obj)
+		{
+			if (obj is X { C: null } x)
+			{
+				Console.WriteLine("Test " + x);
+			}
+			else
+			{
+				Console.WriteLine("not Test");
+			}
+		}
+
+#endif
 		private bool F()
 		{
 			return true;

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -4546,14 +4546,14 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				left = left.UnwrapChild(castExpr.Expression);
 			}
-			var right = TranslatePattern(inst);
+			var right = TranslatePattern(inst, left.Type);
 
 			return new BinaryOperatorExpression(left, BinaryOperatorType.IsPattern, right)
 				.WithRR(new ResolveResult(compilation.FindType(KnownTypeCode.Boolean)))
 				.WithILInstruction(inst);
 		}
 
-		ExpressionWithILInstruction TranslatePattern(ILInstruction pattern)
+		ExpressionWithILInstruction TranslatePattern(ILInstruction pattern, IType leftHandType)
 		{
 			switch (pattern)
 			{
@@ -4590,7 +4590,7 @@ namespace ICSharpCode.Decompiler.CSharp
 								continue;
 							}
 							recursivePatternExpression.SubPatterns.Add(
-								new NamedArgumentExpression { Name = member.Name, Expression = TranslatePattern(subPattern) }
+								new NamedArgumentExpression { Name = member.Name, Expression = TranslatePattern(subPattern, member.ReturnType) }
 									.WithRR(new MemberResolveResult(null, member))
 							);
 						}
@@ -4616,6 +4616,31 @@ namespace ICSharpCode.Decompiler.CSharp
 						return new TypeReferenceExpression(ConvertType(matchInstruction.Variable.Type))
 							.WithILInstruction(matchInstruction);
 					}
+				case Comp comp:
+					var constantValue = Translate(comp.Right, leftHandType);
+					switch (comp.Kind)
+					{
+						case ComparisonKind.Equality:
+							return constantValue
+								.WithILInstruction(comp);
+						case ComparisonKind.Inequality:
+							return new UnaryOperatorExpression(UnaryOperatorType.PatternNot, constantValue)
+								.WithILInstruction(comp);
+						case ComparisonKind.LessThan:
+							return new UnaryOperatorExpression(UnaryOperatorType.PatternRelationalLessThan, constantValue)
+								.WithILInstruction(comp);
+						case ComparisonKind.LessThanOrEqual:
+							return new UnaryOperatorExpression(UnaryOperatorType.PatternRelationalLessThanOrEqual, constantValue)
+								.WithILInstruction(comp);
+						case ComparisonKind.GreaterThan:
+							return new UnaryOperatorExpression(UnaryOperatorType.PatternRelationalGreaterThan, constantValue)
+								.WithILInstruction(comp);
+						case ComparisonKind.GreaterThanOrEqual:
+							return new UnaryOperatorExpression(UnaryOperatorType.PatternRelationalGreaterThanOrEqual, constantValue)
+								.WithILInstruction(comp);
+						default:
+							throw new InvalidOperationException("Unexpected comparison kind: " + comp.Kind);
+					}
 				default:
 					throw new NotImplementedException();
 			}

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/UnaryOperatorExpression.cs

@@ -46,6 +46,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public readonly static TokenRole NullConditionalRole = new TokenRole("?");
 		public readonly static TokenRole SuppressNullableWarningRole = new TokenRole("!");
 		public readonly static TokenRole IndexFromEndRole = new TokenRole("^");
+		public readonly static TokenRole PatternNotRole = new TokenRole("not");
 
 		public UnaryOperatorExpression()
 		{
@@ -126,6 +127,16 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					return SuppressNullableWarningRole;
 				case UnaryOperatorType.IndexFromEnd:
 					return IndexFromEndRole;
+				case UnaryOperatorType.PatternNot:
+					return PatternNotRole;
+				case UnaryOperatorType.PatternRelationalLessThan:
+					return BinaryOperatorExpression.LessThanRole;
+				case UnaryOperatorType.PatternRelationalLessThanOrEqual:
+					return BinaryOperatorExpression.LessThanOrEqualRole;
+				case UnaryOperatorType.PatternRelationalGreaterThan:
+					return BinaryOperatorExpression.GreaterThanRole;
+				case UnaryOperatorType.PatternRelationalGreaterThanOrEqual:
+					return BinaryOperatorExpression.GreaterThanOrEqualRole;
 				default:
 					throw new NotSupportedException("Invalid value for UnaryOperatorType");
 			}
@@ -156,6 +167,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				case UnaryOperatorType.Await:
 				case UnaryOperatorType.SuppressNullableWarning:
 				case UnaryOperatorType.IndexFromEnd:
+				case UnaryOperatorType.PatternNot:
+				case UnaryOperatorType.PatternRelationalLessThan:
+				case UnaryOperatorType.PatternRelationalLessThanOrEqual:
+				case UnaryOperatorType.PatternRelationalGreaterThan:
+				case UnaryOperatorType.PatternRelationalGreaterThanOrEqual:
 					return ExpressionType.Extension;
 				default:
 					throw new NotSupportedException("Invalid value for UnaryOperatorType");
@@ -216,5 +232,25 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// C# 8 prefix ^ operator
 		/// </summary>
 		IndexFromEnd,
+		/// <summary>
+		/// C# 9 not pattern
+		/// </summary>
+		PatternNot,
+		/// <summary>
+		/// C# 9 relational pattern
+		/// </summary>
+		PatternRelationalLessThan,
+		/// <summary>
+		/// C# 9 relational pattern
+		/// </summary>
+		PatternRelationalLessThanOrEqual,
+		/// <summary>
+		/// C# 9 relational pattern
+		/// </summary>
+		PatternRelationalGreaterThan,
+		/// <summary>
+		/// C# 9 relational pattern
+		/// </summary>
+		PatternRelationalGreaterThanOrEqual,
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/PatternMatchingTransform.cs

@@ -183,17 +183,17 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 			if (trueInst.MatchBranch(out var trueBlock) && trueBlock.IncomingEdgeCount == 1 && trueBlock.Parent == container)
 			{
-				DetectPropertySubPatterns((MatchInstruction)ifInst.Condition, trueBlock, falseInst);
+				DetectPropertySubPatterns((MatchInstruction)ifInst.Condition, trueBlock, falseInst, context);
 			}
 
 			return true;
 		}
 
-		private bool DetectPropertySubPatterns(MatchInstruction parentPattern, Block block, ILInstruction parentFalseInst)
+		private bool DetectPropertySubPatterns(MatchInstruction parentPattern, Block block, ILInstruction parentFalseInst, ILTransformContext context)
 		{
 			// if (match.notnull.type[System.String] (V_0 = callvirt get_C(ldloc V_2))) br IL_0022
 			// br IL_0037
-			if (block.Instructions.Count == 2 && block.MatchIfAtEndOfBlock(out var condition, out var trueInst, out var falseInst))
+			if (MatchBlockContainingOneCondition(block, out var condition, out var trueInst, out var falseInst))
 			{
 				if (MatchInstruction.IsPatternMatch(condition, out var operand))
 				{
@@ -213,17 +213,44 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					}
 					if (!DetectExitPoints.CompatibleExitInstruction(parentFalseInst, falseInst))
 					{
-						return false;
+						if (!DetectExitPoints.CompatibleExitInstruction(parentFalseInst, trueInst))
+						{
+							return false;
+						}
+						ExtensionMethods.Swap(ref trueInst, ref falseInst);
+						condition = Comp.LogicNot(condition);
 					}
+					context.Step("Move property sub pattern", condition);
 					parentPattern.SubPatterns.Add(condition);
-					block.Instructions.RemoveAt(0);
-					block.Instructions[0] = trueInst;
+					block.Instructions.Clear();
+					block.Instructions.Add(trueInst);
 					return true;
 				}
 			}
 			return false;
 		}
 
+		private static bool MatchBlockContainingOneCondition(Block block, [NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
+		{
+			switch (block.Instructions.Count)
+			{
+				case 2:
+					return block.MatchIfAtEndOfBlock(out condition, out trueInst, out falseInst);
+				case 3:
+					condition = null;
+					if (!block.MatchIfAtEndOfBlock(out var loadTemp, out trueInst, out falseInst))
+						return false;
+					if (!(loadTemp.MatchLdLoc(out var tempVar) && tempVar.IsSingleDefinition && tempVar.LoadCount == 1))
+						return false;
+					return block.Instructions[0].MatchStLoc(tempVar, out condition);
+				default:
+					condition = null;
+					trueInst = null;
+					falseInst = null;
+					return false;
+			}
+		}
+
 		private bool CheckAllUsesDominatedBy(ILVariable v, BlockContainer container, ILInstruction trueInst,
 			ILInstruction storeToV, ILInstruction? loadInNullCheck, ILTransformContext context, ref ControlFlowGraph? cfg)
 		{