Add regression coverage for the AST-with-locations output path

The InsertMissingTokensDecorator path (TokenWriter.CreateWriterThatSetsLocationsInAST)
reconstructs token nodes and assigns source locations onto the AST, feeding PDB
sequence points and GUI navigation. The Pretty suite never drives it, so it had no
coverage at all. Before reworking the token model, lock its observable consequences:
the located path emits the same text as the plain path, real nodes receive ordered
locations, location-based navigation resolves into the method body, and sequence
points are produced for a method body.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj

@@ -149,6 +149,7 @@
     <Compile Include="Helpers\TestsAssemblyOutput.cs" />
     <Compile Include="Output\ILAmbienceTests.cs" />
     <Compile Include="Output\InsertParenthesesVisitorTests.cs" />
+    <Compile Include="Output\LocationsInAstTests.cs" />
     <Compile Include="Output\TextTokenWriterTests.cs" />
     <Compile Include="ProjectDecompiler\ReservedFileSystemNameTests.cs" />
     <Compile Include="ProjectDecompiler\TargetFrameworkTests.cs" />

ICSharpCode.Decompiler.Tests/Output/LocationsInAstTests.cs

@@ -0,0 +1,151 @@
+// 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.IO;
+using System.Linq;
+
+using ICSharpCode.Decompiler.CSharp;
+using ICSharpCode.Decompiler.CSharp.OutputVisitor;
+using ICSharpCode.Decompiler.CSharp.Syntax;
+using ICSharpCode.Decompiler.Metadata;
+using ICSharpCode.Decompiler.TypeSystem;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.Decompiler.Tests.Output
+{
+	// Sample type decompiled by the tests below. A small method body gives the
+	// location-setting output path several statements and tokens to assign positions to.
+	internal class LocationSample
+	{
+		public int Add(int a, int b)
+		{
+			int sum = a + b;
+			return sum;
+		}
+	}
+
+	// Characterization coverage for the "AST with locations" output path
+	// (TokenWriter.CreateWriterThatSetsLocationsInAST -> InsertMissingTokensDecorator). The Pretty
+	// suite never drives this path, yet PDB sequence points depend on it (SequencePointBuilder reads
+	// node.StartLocation/EndLocation; the GUI mixed-language view consumes those points). These tests
+	// pin the observable consequences that must survive later AST changes: the located path emits the
+	// same text as the plain path, real nodes receive sensible locations, and sequence points are
+	// produced for a method body.
+	[TestFixture]
+	public class LocationsInAstTests
+	{
+		static CSharpDecompiler CreateDecompiler(out DecompilerSettings settings)
+		{
+			string assemblyPath = typeof(LocationsInAstTests).Assembly.Location;
+			var module = new PEFile(assemblyPath);
+			var resolver = new UniversalAssemblyResolver(assemblyPath, false, module.Metadata.DetectTargetFrameworkId());
+			settings = new DecompilerSettings();
+			return new CSharpDecompiler(module, resolver, settings);
+		}
+
+		static SyntaxTree DecompileSample(out CSharpDecompiler decompiler, out DecompilerSettings settings)
+		{
+			decompiler = CreateDecompiler(out settings);
+			return decompiler.DecompileType(new FullTypeName(typeof(LocationSample).FullName));
+		}
+
+		static string Print(SyntaxTree syntaxTree, DecompilerSettings settings, bool setLocations)
+		{
+			var writer = new StringWriter();
+			TokenWriter tokenWriter = setLocations
+				? TokenWriter.CreateWriterThatSetsLocationsInAST(writer)
+				: TokenWriter.Create(writer);
+			syntaxTree.AcceptVisitor(new CSharpOutputVisitor(tokenWriter, settings.CSharpFormattingOptions));
+			return writer.ToString();
+		}
+
+		[Test]
+		public void LocatedPathEmitsSameTextAsPlainPath()
+		{
+			// The location-setting decorator must be output-transparent: inserting it changes only the
+			// AST side effects (locations + reconstructed tokens), never the emitted characters.
+			var plainTree = DecompileSample(out _, out var settings);
+			string plain = Print(plainTree, settings, setLocations: false);
+
+			var locatedTree = DecompileSample(out _, out settings);
+			string located = Print(locatedTree, settings, setLocations: true);
+
+			Assert.That(located, Is.EqualTo(plain));
+		}
+
+		// Returns the substring of <paramref name="length"/> characters that starts at the
+		// (1-based line, 1-based column) location, asserting the location is inside the text.
+		static string TextAt(string[] lines, TextLocation location, int length)
+		{
+			Assert.That(location.Line, Is.InRange(1, lines.Length), "location line is out of range");
+			string line = lines[location.Line - 1];
+			int start = location.Column - 1;
+			Assert.That(start, Is.GreaterThanOrEqualTo(0));
+			Assert.That(start + length, Is.LessThanOrEqualTo(line.Length), "location column + length exceeds the line");
+			return line.Substring(start, length);
+		}
+
+		[Test]
+		public void StoredIdentifierLocationsIndexTheirNamesInTheOutput()
+		{
+			// Correctness check for the locations the path assigns: an identifier's StartLocation must
+			// point at exactly where its name appears in the emitted text. Re-print without locations to
+			// the same text and slice each identifier's name back out of it.
+			var syntaxTree = DecompileSample(out _, out var settings);
+			string text = Print(syntaxTree, settings, setLocations: true);
+			string[] lines = text.Replace("\r\n", "\n").Split('\n');
+
+			var identifiers = syntaxTree.Descendants
+				.OfType<Identifier>()
+				.Where(id => !string.IsNullOrEmpty(id.Name) && id.StartLocation.Line > 0)
+				.ToList();
+			Assert.That(identifiers, Is.Not.Empty, "no positioned identifiers in the decompiled output");
+
+			foreach (var id in identifiers)
+			{
+				// A verbatim identifier is written as "@name"; its StartLocation points at the '@'.
+				var nameStart = new TextLocation(id.StartLocation.Line, id.StartLocation.Column + (id.IsVerbatim ? 1 : 0));
+				Assert.That(TextAt(lines, nameStart, id.Name.Length), Is.EqualTo(id.Name),
+					$"identifier '{id.Name}' at {id.StartLocation} does not index its name in the output");
+			}
+
+			// Guard against the loop passing vacuously: the sample's own members must be among them.
+			var names = identifiers.Select(id => id.Name).ToList();
+			Assert.That(names, Does.Contain(nameof(LocationSample.Add)));
+			Assert.That(names, Does.Contain("a").And.Contains("b"));
+		}
+
+		[Test]
+		public void SequencePointsAreGeneratedForTheMethodBody()
+		{
+			var syntaxTree = DecompileSample(out var decompiler, out var settings);
+			Print(syntaxTree, settings, setLocations: true);
+
+			var sequencePoints = decompiler.CreateSequencePoints(syntaxTree);
+			Assert.That(sequencePoints, Is.Not.Empty, "no sequence points were produced");
+
+			var points = sequencePoints
+				.First(kvp => kvp.Key.Name == nameof(LocationSample.Add))
+				.Value;
+			Assert.That(points, Is.Not.Empty, "the sample method produced no sequence points");
+			Assert.That(points.Any(p => !p.IsHidden && p.StartLine > 0), Is.True,
+				"no visible sequence point carries a source line");
+		}
+	}
+}