Support optional children in roles and generated DoMatch

Preparation for making optional single-value slots nullable: a role may omit
its null object and a slot may be cleared via SetChildByRole(role, null), and
the generated DoMatch emits MatchOptional for a nullable single child so an
absent child matches correctly. Both are inert until a slot is actually made
nullable.
Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler.Generators/DecompilerSyntaxTreeGenerator.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.Decompiler.Generators;
 [Generator]
 internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 {
-	record AstNodeAdditions(string NodeName, bool NeedsAcceptImpls, bool NeedsVisitor, bool NeedsNullNode, bool NeedsPatternPlaceholder, int NullNodeBaseCtorParamCount, bool IsTypeNode, string VisitMethodName, string VisitMethodParamType, EquatableArray<(string Member, string TypeName, bool RecursiveMatch, bool MatchAny)>? MembersToMatch, EquatableArray<(string RoleExpr, bool IsCollection, string PropertyName, string PropertyType, string ElementType, bool IsOverride, bool IsNullable, string KindName)>? Slots);
+	record AstNodeAdditions(string NodeName, bool NeedsAcceptImpls, bool NeedsVisitor, bool NeedsNullNode, bool NeedsPatternPlaceholder, int NullNodeBaseCtorParamCount, bool IsTypeNode, string VisitMethodName, string VisitMethodParamType, EquatableArray<(string Member, string TypeName, bool RecursiveMatch, bool MatchAny, bool Nullable)>? MembersToMatch, EquatableArray<(string RoleExpr, bool IsCollection, string PropertyName, string PropertyType, string ElementType, bool IsOverride, bool IsNullable, string KindName)>? Slots);
 
 	// Derives the shared SlotKind name from a [Slot] role expression: the last dotted segment with a
 	// trailing "Role" removed (e.g. "Roles.EmbeddedStatement" -> "EmbeddedStatement", "LeftRole" ->
@@ -56,7 +56,7 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 			_ => (targetSymbol.Name, targetSymbol.Name),
 		};
 
-		List<(string Member, string TypeName, bool RecursiveMatch, bool MatchAny)>? membersToMatch = null;
+		List<(string Member, string TypeName, bool RecursiveMatch, bool MatchAny, bool Nullable)>? membersToMatch = null;
 
 		if (!targetSymbol.MemberNames.Contains("DoMatch"))
 		{
@@ -65,7 +65,7 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 			var astNodeType = (INamedTypeSymbol)context.SemanticModel.GetSpeculativeSymbolInfo(context.TargetNode.Span.Start, SyntaxFactory.ParseTypeName("AstNode"), SpeculativeBindingOption.BindAsTypeOrNamespace).Symbol!;
 
 			if (targetSymbol.BaseType!.MemberNames.Contains("MatchAttributesAndModifiers"))
-				membersToMatch.Add(("MatchAttributesAndModifiers", null!, false, false));
+				membersToMatch.Add(("MatchAttributesAndModifiers", null!, false, false, false));
 
 			foreach (var m in targetSymbol.GetMembers())
 			{
@@ -75,16 +75,17 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 					continue;
 				if (property.Type.MetadataName is "CSharpTokenNode" or "TextLocation")
 					continue;
+				bool nullable = property.NullableAnnotation == NullableAnnotation.Annotated;
 				switch (property.Type)
 				{
 					case INamedTypeSymbol named when named.IsDerivedFrom(astNodeType) || named.MetadataName == "AstNodeCollection`1":
-						membersToMatch.Add((property.Name, named.Name, true, false));
+						membersToMatch.Add((property.Name, named.Name, true, false, nullable));
 						break;
 					case INamedTypeSymbol { TypeKind: TypeKind.Enum } named when named.GetMembers().Any(_ => _.Name == "Any"):
-						membersToMatch.Add((property.Name, named.Name, false, true));
+						membersToMatch.Add((property.Name, named.Name, false, true, false));
 						break;
 					default:
-						membersToMatch.Add((property.Name, property.Type.Name, false, false));
+						membersToMatch.Add((property.Name, property.Type.Name, false, false, false));
 						break;
 				}
 			}
@@ -278,12 +279,17 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 	{{
 		return other is {source.NodeName} o && !o.IsNull");
 
-			foreach (var (member, typeName, recursive, hasAny) in source.MembersToMatch)
+			foreach (var (member, typeName, recursive, hasAny, nullable) in source.MembersToMatch)
 			{
 				if (member == "MatchAttributesAndModifiers")
 				{
 					builder.Append($"\r\n\t\t\t&& this.MatchAttributesAndModifiers(o, match)");
 				}
+				else if (recursive && nullable && typeName != "AstNodeCollection")
+				{
+					// An optional single-value child is null when absent; match null-safely.
+					builder.Append($"\r\n\t\t\t&& MatchOptional(this.{member}, o.{member}, match)");
+				}
 				else if (recursive)
 				{
 					builder.Append($"\r\n\t\t\t&& this.{member}.DoMatch(o.{member}, match)");

ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs

@@ -383,7 +383,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return collection != null ? (AstNodeCollection<T>)collection : new AstNodeCollection<T>(this, role);
 		}
 
-		protected void SetChildByRole<T>(Role<T> role, T newChild) where T : AstNode
+		protected void SetChildByRole<T>(Role<T> role, T? newChild) where T : AstNode
 		{
 			SetChildByRoleUntyped(role, newChild);
 		}
@@ -724,6 +724,16 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return PatternMatching.Pattern.MatchString(pattern, text);
 		}
 
+		/// <summary>
+		/// Matches two optional children: both absent (null), or both present and matching.
+		/// When the pattern side is present, its own DoMatch decides (e.g. an OptionalNode matches an
+		/// absent candidate). When it is absent, the candidate must be absent too.
+		/// </summary>
+		protected static bool MatchOptional(AstNode? thisChild, AstNode? otherChild, PatternMatching.Match match)
+		{
+			return thisChild != null ? thisChild.DoMatch(otherChild, match) : otherChild == null;
+		}
+
 		protected internal abstract bool DoMatch(AstNode? other, PatternMatching.Match match);
 
 		bool PatternMatching.INode.DoMatch(PatternMatching.INode? other, PatternMatching.Match match)

ICSharpCode.Decompiler/CSharp/Syntax/Role.cs

@@ -96,10 +96,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return node is T;
 		}
 
-		public Role(string name, T nullObject)
+		public Role(string name, T? nullObject)
 		{
 			this.name = name ?? throw new ArgumentNullException(nameof(name));
-			this.nullObject = nullObject;
+			// Roles used only for collections may omit the null object (pass null); NullObject then
+			// returns null, matching the documented contract. Single-value roles always pass one.
+			this.nullObject = nullObject!;
 		}
 
 		public override string ToString()