Detect local functions renamed by obfuscators

Obfuscators strip the CompilerGeneratedAttribute and rewrite the
"<caller>g__name|x_y" name, which is all IsLocalFunctionMethod had to go on.
The method then stays an ordinary static method, its display struct escapes by
ref into a plain call, and TransformDisplayClassUsage correctly refuses to
scalar-replace it -- so the closure fields leak into the output as
"<>c__DisplayClass29_0_.iid" (issue #3202).

The one marker an obfuscator cannot remove is the signature: Roslyn emits
struct closures exclusively for local functions, and no hand-written C# can
name a "<>c__DisplayClass" type, so a by-ref parameter of one identifies the
method regardless of what it is called.

#3202

Assisted-by: Claude:claude-opus-5[1m]:Claude Code

ICSharpCode.Decompiler/IL/Transforms/LocalFunctionDecompiler.cs

@@ -781,36 +781,46 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			var method = metadata.GetMethodDefinition(methodHandle);
 			var declaringType = method.GetDeclaringType();
 
-			if ((method.Attributes & MethodAttributes.Assembly) == 0 || !(method.IsCompilerGenerated(metadata) || declaringType.IsCompilerGenerated(metadata)))
+			if ((method.Attributes & MethodAttributes.Assembly) == 0)
 				return false;
 
-			if (!ParseLocalFunctionName(metadata.GetString(method.Name), out _, out _))
-				return false;
+			if ((method.IsCompilerGenerated(metadata) || declaringType.IsCompilerGenerated(metadata))
+				&& ParseLocalFunctionName(metadata.GetString(method.Name), out _, out _))
+			{
+				return true;
+			}
 
-			return true;
+			// Obfuscators strip the CompilerGeneratedAttribute and rewrite the
+			// "<caller>g__name|x_y" name, but they cannot remove the by-ref display-struct
+			// parameter: a compiler-generated struct closure is only ever passed by reference
+			// to the local functions that capture it.
+			return HasDisplayStructParameter(module, methodHandle);
 		}
 
-		public static bool LocalFunctionNeedsAccessibilityChange(MetadataFile module, MethodDefinitionHandle methodHandle)
+		/// <summary>
+		/// True if any parameter is a by-ref compiler-generated closure struct of this module.
+		/// </summary>
+		static bool HasDisplayStructParameter(MetadataFile module, MethodDefinitionHandle methodHandle)
 		{
-			if (!IsLocalFunctionMethod(module, methodHandle))
-				return false;
-
 			var metadata = module.Metadata;
 			var method = metadata.GetMethodDefinition(methodHandle);
-
 			FindRefStructParameters visitor = new FindRefStructParameters();
 			method.DecodeSignature(visitor, default);
-
 			foreach (var h in visitor.RefStructTypes)
 			{
 				var td = metadata.GetTypeDefinition(h);
-				if (td.IsCompilerGenerated(metadata) && td.IsValueType(metadata))
+				if (td.IsCompilerGenerated(metadata) && td.IsValueType(metadata) && td.HasGeneratedName(metadata))
 					return true;
 			}
-
 			return false;
 		}
 
+		public static bool LocalFunctionNeedsAccessibilityChange(MetadataFile module, MethodDefinitionHandle methodHandle)
+		{
+			return IsLocalFunctionMethod(module, methodHandle)
+				&& HasDisplayStructParameter(module, methodHandle);
+		}
+
 		public static bool IsLocalFunctionDisplayClass(MetadataFile module, TypeDefinitionHandle typeHandle, ILTransformContext context = null)
 		{
 			if (context != null && context.PEFile != module)