Do not fall back to a sibling's documentation key

The C#/Roslyn-form ID of a member whose C++/CLI form differs can equal
the only key of a same-named sibling overload (char* vs signed char*).
Assemblies containing such overloads cannot come from the C# compiler,
so their xml files use the C++/CLI dialect, where that key documents
the sibling: falling back to the Roslyn form would show the sibling's
documentation for an undocumented member. GetIdStringCandidates now
omits the Roslyn form when a same-named sibling's C++/CLI form owns it,
so a lookup miss stays a miss.

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

ICSharpCode.Decompiler.Tests/Documentation/IdStringProviderTests.cs

@@ -2119,6 +2119,60 @@ namespace ModreqParams
 			Assert.That(modifiedHandle, Is.EqualTo((EntityHandle)MetadataTokens.MethodDefinitionHandle(1)));
 		}
 
+		[Test]
+		public void DocumentationLookup_DoesNotFallBackToSiblingKey()
+		{
+			// The Roslyn form of the modified overload equals the only key of the
+			// unmodified overload. Such an assembly cannot come from the C# compiler, so
+			// its xml file uses the C++/CLI dialect, where that key documents the
+			// unmodified overload: the modified overload's candidates must omit it, and
+			// its documentation lookup must miss instead of showing the sibling's text.
+			var pe = BuildAssemblyWithMethods(
+				(metadata, parameter) => {
+					var pointee = parameter.Type().Pointer();
+					pointee.CustomModifiers().AddModifier(
+						AddCompilerServicesTypeRef(metadata, "IsSignUnspecifiedByte"), isOptional: true);
+					pointee.SByte();
+				},
+				(metadata, parameter) => parameter.Type().Pointer().SByte());
+
+			Assert.That(pe.GetIdStringCandidates(MetadataTokens.MethodDefinitionHandle(1)),
+				Is.EqualTo(new[] { "M:Host.M(System.SByte!System.Runtime.CompilerServices.IsSignUnspecifiedByte*)" }));
+			Assert.That(pe.GetIdStringCandidates(MetadataTokens.MethodDefinitionHandle(2)),
+				Is.EqualTo(new[] { "M:Host.M(System.SByte*)" }));
+
+			string xmlPath = Path.Combine(Path.GetTempPath(),
+				"IdStringSiblingGuard_" + Guid.NewGuid().ToString("N") + ".xml");
+			File.WriteAllText(xmlPath, """
+				<?xml version="1.0"?>
+				<doc>
+					<assembly><name>test</name></assembly>
+					<members>
+						<member name="M:Host.M(System.SByte*)">
+							<summary>plain overload</summary>
+						</member>
+					</members>
+				</doc>
+				""");
+			try
+			{
+				var provider = new XmlDocumentationProvider(xmlPath);
+				var compilation = new SimpleCompilation(pe, MinimalCorlib.Instance);
+				var host = compilation.MainModule.TopLevelTypeDefinitions.Single(t => t.Name == "Host");
+				var modified = host.Methods.Single(
+					m => m.MetadataToken == (EntityHandle)MetadataTokens.MethodDefinitionHandle(1));
+				var plain = host.Methods.Single(
+					m => m.MetadataToken == (EntityHandle)MetadataTokens.MethodDefinitionHandle(2));
+
+				Assert.That(provider.GetDocumentation(plain), Does.Contain("plain overload"));
+				Assert.That(provider.GetDocumentation(modified), Is.Null);
+			}
+			finally
+			{
+				File.Delete(xmlPath);
+			}
+		}
+
 		#endregion
 
 		#region MSVC C++/CLI dialect fixture

ICSharpCode.Decompiler/Documentation/IdStringProvider.cs

@@ -56,15 +56,76 @@ namespace ICSharpCode.Decompiler.Documentation
 		/// it contains character sequences Roslyn never writes, so it can only match
 		/// MSVC-generated keys; the stripped Roslyn form of one member can collide with the
 		/// key of a different member in an MSVC-generated file (e.g. overloads differing
-		/// only in a custom modifier).
+		/// only in a custom modifier). When such a colliding sibling overload exists, the
+		/// Roslyn form is omitted entirely: assemblies containing such overloads cannot
+		/// come from the C# compiler, so their xml files use the C++/CLI dialect, where
+		/// that key documents the sibling.
 		/// </summary>
 		public static IEnumerable<string> GetIdStringCandidates(this MetadataFile module, EntityHandle handle)
 		{
 			string primary = GetIdString(module, handle, cppCliDialect: false);
 			string cppCli = GetIdString(module, handle, cppCliDialect: true);
-			if (cppCli != primary)
-				yield return cppCli;
-			yield return primary;
+			if (cppCli == primary)
+			{
+				yield return primary;
+				yield break;
+			}
+			yield return cppCli;
+			if (!RoslynFormBelongsToSibling(module, handle, primary))
+				yield return primary;
+		}
+
+		/// <summary>
+		/// True when the C#/Roslyn-form ID of <paramref name="handle"/> equals the
+		/// C++/CLI-form ID of a same-named sibling member of the same type (only members
+		/// with a signature portion can diverge, so only methods and properties are
+		/// checked). Keys embed the declaring type and member name, so no other member
+		/// can own the string.
+		/// </summary>
+		static bool RoslynFormBelongsToSibling(MetadataFile module, EntityHandle handle, string roslynForm)
+		{
+			var metadata = module.Metadata;
+			switch (handle.Kind)
+			{
+				case HandleKind.MethodDefinition:
+				{
+					var methodHandle = (MethodDefinitionHandle)handle;
+					var methodDef = metadata.GetMethodDefinition(methodHandle);
+					string name = metadata.GetString(methodDef.Name);
+					foreach (var sibling in metadata.GetTypeDefinition(methodDef.GetDeclaringType()).GetMethods())
+					{
+						if (sibling == methodHandle
+							|| !metadata.StringComparer.Equals(metadata.GetMethodDefinition(sibling).Name, name))
+						{
+							continue;
+						}
+						if (GetIdString(module, sibling, cppCliDialect: true) == roslynForm)
+							return true;
+					}
+					return false;
+				}
+
+				case HandleKind.PropertyDefinition:
+				{
+					var propertyHandle = (PropertyDefinitionHandle)handle;
+					string name = metadata.GetString(metadata.GetPropertyDefinition(propertyHandle).Name);
+					var declaringType = FindDeclaringTypeOfProperty(metadata, propertyHandle);
+					foreach (var sibling in metadata.GetTypeDefinition(declaringType).GetProperties())
+					{
+						if (sibling == propertyHandle
+							|| !metadata.StringComparer.Equals(metadata.GetPropertyDefinition(sibling).Name, name))
+						{
+							continue;
+						}
+						if (GetIdString(module, sibling, cppCliDialect: true) == roslynForm)
+							return true;
+					}
+					return false;
+				}
+
+				default:
+					return false;
+			}
 		}
 
 		static string GetIdString(MetadataFile module, EntityHandle handle, bool cppCliDialect)