Fix #2078: Generic local functions not highlighted properly

LocalFunctionMethod.MemberDefinition returned the instance itself, so the
declaration of a generic local function (whose base method is an identity
specialization) and its use sites (whose base methods carry the use-site
substitutions) never compared equal, and click-highlighting could not group
them. Returning a wrapper around the unspecialized base method lets the
token writer record the same definition object for the declaration and all
use sites. Method-group references to local functions additionally need
their own lookup, because GetSymbol() does not surface a
MethodGroupResolveResult.

The token-writer tests now keep the PEFile alive for the duration of each
test: recorded references are type-system entities that lazily read from
the PE image, and formatting one in an assertion message after disposal
crashed with an AccessViolationException.

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

ICSharpCode.Decompiler.Tests/Output/TextTokenWriterTests.cs

@@ -52,12 +52,37 @@ namespace ICSharpCode.Decompiler.Tests.Output
 		public override void Method() { }
 	}
 
+	// Exercises local-reference output for local functions: the definition and every use site
+	// (specialized generic invocations and method-group references) must be written with equal
+	// reference objects, otherwise click-highlighting in the UI cannot match them (issue #2078).
+	internal class GenericLocalFunctionHost
+	{
+		public int Use()
+		{
+			Func<int, int> f = Identity<int>;
+			return Identity(42) + Identity("a").Length + f(0);
+
+			static T2 Identity<T2>(T2 value) => value;
+		}
+
+		// Inside a generic method, Roslyn prepends the enclosing method's type parameters to the
+		// compiler-generated method backing the local function, so every use site references a
+		// specialized method even when the local function's own type argument is a type parameter.
+		public T UseInGenericMethod<T>(T t)
+		{
+			return Echo(Echo(t));
+
+			static T2 Echo<T2>(T2 value) => value;
+		}
+	}
+
 	[TestFixture]
 	public class TextTokenWriterTests
 	{
 		sealed class ReferenceRecordingOutput : ITextOutput
 		{
 			public readonly List<(string Text, IMember Member)> MemberReferences = new();
+			public readonly List<(string Text, object Reference, bool IsDefinition)> LocalReferences = new();
 			public readonly List<bool> FoldStartDefaultCollapsed = new();
 			public int FoldEndCount;
 
@@ -77,7 +102,10 @@ namespace ICSharpCode.Decompiler.Tests.Output
 					MemberReferences.Add((text, member));
 			}
 
-			public void WriteLocalReference(string text, object reference, bool isDefinition = false) { }
+			public void WriteLocalReference(string text, object reference, bool isDefinition = false)
+			{
+				LocalReferences.Add((text, reference, isDefinition));
+			}
 
 			public void MarkFoldStart(string collapsedText = "...", bool defaultCollapsed = false, bool isDefinition = false)
 			{
@@ -90,11 +118,17 @@ namespace ICSharpCode.Decompiler.Tests.Output
 			}
 		}
 
-		static List<(string Text, IMember Member)> DecompileAndCollectMemberReferences(Type type)
+		// The module must stay alive until the caller is done with the recorded reference
+		// objects: they are type-system entities that lazily read from the PE image, e.g.
+		// when NUnit formats an assertion message.
+		static PEFile OpenTestAssembly()
+		{
+			return new PEFile(typeof(TextTokenWriterTests).Assembly.Location);
+		}
+
+		static ReferenceRecordingOutput DecompileAndCollectReferences(PEFile module, Type type)
 		{
-			string assemblyPath = typeof(TextTokenWriterTests).Assembly.Location;
-			using var module = new PEFile(assemblyPath);
-			var resolver = new UniversalAssemblyResolver(assemblyPath, false, module.Metadata.DetectTargetFrameworkId());
+			var resolver = new UniversalAssemblyResolver(module.FileName, false, module.Metadata.DetectTargetFrameworkId());
 			var settings = new DecompilerSettings();
 			var decompiler = new CSharpDecompiler(module, resolver, settings);
 			var syntaxTree = decompiler.DecompileType(new FullTypeName(type.FullName));
@@ -102,7 +136,7 @@ namespace ICSharpCode.Decompiler.Tests.Output
 			var output = new ReferenceRecordingOutput();
 			var tokenWriter = new TextTokenWriter(output, settings);
 			syntaxTree.AcceptVisitor(new CSharpOutputVisitor(tokenWriter, settings.CSharpFormattingOptions));
-			return output.MemberReferences;
+			return output;
 		}
 
 		static string FormatMember(IMember member) => $"{member.DeclaringType?.Name}.{member.Name}";
@@ -110,7 +144,8 @@ namespace ICSharpCode.Decompiler.Tests.Output
 		[Test]
 		public void OverrideKeywordReferencesTheOverriddenBaseMember()
 		{
-			var references = DecompileAndCollectMemberReferences(typeof(OverrideLinkDerived));
+			using var module = OpenTestAssembly();
+			var references = DecompileAndCollectReferences(module, typeof(OverrideLinkDerived)).MemberReferences;
 
 			var overrideTargets = references.Where(r => r.Text == "override").Select(r => FormatMember(r.Member));
 			Assert.That(overrideTargets, Is.EquivalentTo(new[] {
@@ -122,7 +157,8 @@ namespace ICSharpCode.Decompiler.Tests.Output
 		[Test]
 		public void OverrideKeywordReferencesTheNearestOverriddenMember()
 		{
-			var references = DecompileAndCollectMemberReferences(typeof(OverrideLinkSecondLevel));
+			using var module = OpenTestAssembly();
+			var references = DecompileAndCollectReferences(module, typeof(OverrideLinkSecondLevel)).MemberReferences;
 
 			var overrideTargets = references.Where(r => r.Text == "override").Select(r => FormatMember(r.Member));
 			Assert.That(overrideTargets, Is.EquivalentTo(new[] {
@@ -133,7 +169,8 @@ namespace ICSharpCode.Decompiler.Tests.Output
 		[Test]
 		public void VirtualAndAbstractModifiersAreNotReferences()
 		{
-			var references = DecompileAndCollectMemberReferences(typeof(OverrideLinkBase));
+			using var module = OpenTestAssembly();
+			var references = DecompileAndCollectReferences(module, typeof(OverrideLinkBase)).MemberReferences;
 
 			Assert.That(references.Where(r => r.Text is "virtual" or "abstract" or "override"), Is.Empty);
 		}
@@ -192,5 +229,45 @@ namespace ICSharpCode.Decompiler.Tests.Output
 			Assert.That(output.FoldStartDefaultCollapsed, Has.Count.EqualTo(1));
 			Assert.That(output.FoldEndCount, Is.EqualTo(1));
 		}
+
+		[Test]
+		public void GenericLocalFunctionDefinitionAndUsesShareEqualReferenceObjects()
+		{
+			using var module = OpenTestAssembly();
+			var output = DecompileAndCollectReferences(module, typeof(GenericLocalFunctionHost));
+
+			Assert.Multiple(() => {
+				// Two invocations (with different inferred type arguments) and one method-group reference.
+				AssertLocalFunctionReferenceGroup(output, "Identity", expectedUses: 3);
+				// Two nested invocations whose type argument is the enclosing method's type parameter.
+				AssertLocalFunctionReferenceGroup(output, "Echo", expectedUses: 2);
+			});
+		}
+
+		// Runs inside Assert.Multiple: after a recorded assertion failure, bail out of the checks
+		// that would dereference the missing data instead of throwing.
+		static void AssertLocalFunctionReferenceGroup(ReferenceRecordingOutput output, string name, int expectedUses)
+		{
+			var references = output.LocalReferences.Where(r => r.Text == name).ToList();
+
+			var definitions = references.Where(r => r.IsDefinition).ToList();
+			Assert.That(definitions, Has.Count.EqualTo(1), $"{name}: definition count");
+			if (definitions.Count != 1)
+				return;
+			object definition = definitions[0].Reference;
+			Assert.That(definition, Is.InstanceOf<IMethod>(), $"{name}: definition reference type");
+			if (definition is not IMethod method)
+				return;
+			Assert.That(method.IsLocalFunction, Is.True, $"{name}: IsLocalFunction");
+
+			var uses = references.Where(r => !r.IsDefinition).ToList();
+			Assert.That(uses, Has.Count.EqualTo(expectedUses), $"{name}: use count");
+			foreach (var use in uses)
+			{
+				Assert.That(use.Reference, Is.EqualTo(definition), $"{name}: use equals definition");
+				Assert.That(definition, Is.EqualTo(use.Reference), $"{name}: definition equals use");
+				Assert.That(use.Reference.GetHashCode(), Is.EqualTo(definition.GetHashCode()), $"{name}: hash codes");
+			}
+		}
 	}
 }

ICSharpCode.Decompiler/Output/TextTokenWriter.cs

@@ -22,6 +22,7 @@ using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp;
 using ICSharpCode.Decompiler.CSharp.OutputVisitor;
+using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.Semantics;
@@ -157,11 +158,23 @@ namespace ICSharpCode.Decompiler
 					return method + gotoStatement.Label;
 			}
 
+			// Local-function references are recorded with the unspecialized definition, so that
+			// generic use sites (which carry specialized methods) and the declaration all share
+			// equal reference objects and can be matched for highlighting.
 			if (node.Slot?.Kind == Slots.TargetExpression && node.Parent is InvocationExpression)
 			{
 				var symbol = node.Parent.GetSymbol();
-				if (symbol is LocalFunctionMethod)
-					return symbol;
+				if (symbol is LocalFunctionMethod localFunction)
+					return localFunction.MemberDefinition;
+			}
+
+			// Method-group references to local functions (delegate conversions) are annotated
+			// with a MethodGroupResolveResult, which GetSymbol() does not surface. A local
+			// function cannot be overloaded, so such a group contains exactly one method.
+			if (node.GetResolveResult() is MethodGroupResolveResult methodGroup
+				&& methodGroup.Methods.FirstOrDefault() is LocalFunctionMethod methodGroupLocalFunction)
+			{
+				return methodGroupLocalFunction.MemberDefinition;
 			}
 
 			return null;
@@ -205,7 +218,7 @@ namespace ICSharpCode.Decompiler
 			{
 				var localFunction = node.Parent.GetResolveResult() as MemberResolveResult;
 				if (localFunction != null)
-					return localFunction.Member;
+					return localFunction.Member.MemberDefinition;
 			}
 
 			return null;

ICSharpCode.Decompiler/TypeSystem/Implementation/LocalFunctionMethod.cs

@@ -79,7 +79,15 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		internal bool IsStaticLocalFunction { get; }
 
-		public IMember MemberDefinition => this;
+		public IMember MemberDefinition {
+			get {
+				var baseMethodDefinition = (IMethod)baseMethod.MemberDefinition;
+				if (ReferenceEquals(baseMethodDefinition, baseMethod))
+					return this;
+				return new LocalFunctionMethod(baseMethodDefinition, Name, IsStaticLocalFunction,
+					NumberOfCompilerGeneratedParameters, NumberOfCompilerGeneratedTypeParameters);
+			}
+		}
 
 		public IType ReturnType => baseMethod.ReturnType;
 		IEnumerable<IMember> IMember.ExplicitlyImplementedInterfaceMembers => baseMethod.ExplicitlyImplementedInterfaceMembers;

ILSpy.Tests/Editor/LocalReferenceHighlightTests.cs

@@ -0,0 +1,83 @@
+// Copyright (c) 2026 Siegfried Pammer
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System.Linq;
+using System.Threading.Tasks;
+
+using Avalonia.Headless.NUnit;
+using Avalonia.VisualTree;
+
+using AwesomeAssertions;
+
+using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.ILSpy.TextView;
+using ICSharpCode.ILSpy.TreeNodes;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.TextView;
+
+/// <summary>
+/// Sample type decompiled by <see cref="LocalReferenceHighlightTests"/>: a generic local
+/// function inside a generic method, so that every use site carries a specialized method
+/// instance while the declaration carries the unspecialized one (issue #2078).
+/// </summary>
+public class GenericLocalFunctionSample
+{
+	public T Run<T>(T t)
+	{
+		return Echo(Echo(t));
+
+		static T2 Echo<T2>(T2 value) => value;
+	}
+}
+
+/// <summary>
+/// Pins the local-reference highlighting for local functions: clicking any occurrence of a
+/// generic local function must paint the definition and every use site as one group.
+/// </summary>
+[TestFixture]
+public class LocalReferenceHighlightTests
+{
+	[AvaloniaTest]
+	public async Task Clicking_A_Generic_Local_Function_Use_Highlights_Definition_And_All_Uses()
+	{
+		var (window, vm) = await TestHarness.BootAsync();
+		await vm.OpenAssemblyAsync(typeof(GenericLocalFunctionSample).Assembly.Location);
+		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
+			"ILSpy.Tests",
+			"ICSharpCode.ILSpy.Tests.TextView",
+			"ICSharpCode.ILSpy.Tests.TextView.GenericLocalFunctionSample");
+		vm.AssemblyTreeModel.SelectNode(typeNode);
+		var tab = await vm.DockWorkspace.WaitForDecompiledTextAsync();
+		var view = window.GetVisualDescendants().OfType<DecompilerTextView>().First();
+
+		var localFunctionSegments = tab.References!
+			.Where(r => r.IsLocal && r.Reference is IMethod { IsLocalFunction: true })
+			.ToList();
+		localFunctionSegments.Should().HaveCount(3, "the local function has one definition and two calls");
+		localFunctionSegments.Count(r => r.IsDefinition).Should().Be(1);
+
+		var use = localFunctionSegments.First(r => !r.IsDefinition);
+		view.OnReferenceClicked(use);
+
+		view.LocalReferenceMarks.Select(m => m.StartOffset).Should().BeEquivalentTo(
+			localFunctionSegments.Select(s => s.StartOffset),
+			"clicking a use must highlight the definition and every use");
+	}
+}

ILSpy/TextView/DecompilerTextView.axaml.cs

@@ -348,6 +348,9 @@ namespace ICSharpCode.ILSpy.TextView
 		/// document, so an HTML copy can include it on top of the xshd syntax colours.</summary>
 		internal RichTextModel? SemanticHighlightingModel => boundModel?.HighlightingModel;
 
+		/// <summary>The currently painted local-reference highlight marks (test observability).</summary>
+		internal IReadOnlyList<TextMarker> LocalReferenceMarks => localReferenceMarks;
+
 		// ThemeManager.Current is a process-lived singleton, so subscribing to its ThemeChanged in
 		// the constructor and never detaching would root every DecompilerTextView for the lifetime of
 		// the process -- one leaked view per decompiler tab. Bind the handler to the visual-tree