[Bytecode] Hide Kotlin synthetic default constructors.

Author: jpobst

src/Xamarin.Android.Tools.Bytecode/Kotlin/KotlinFixups.cs

@@ -93,6 +93,12 @@ static void FixupJavaMethods (Methods methods)
 				method.AccessFlags = MethodAccessFlags.Private;
 			}
 
+			// Hide constructor if it's the synthetic DefaultConstructorMarker one
+			foreach (var method in methods.Where (method => method.IsDefaultConstructorMarker ())) {
+				Log.Debug ($"Kotlin: Hiding synthetic default constructor in class '{method.DeclaringType?.ThisClass.Name.Value}' with signature '{method.Descriptor}'");
+				method.AccessFlags = MethodAccessFlags.Private;
+			}
+
 			// Better parameter names in extension methods
 			foreach (var method in methods.Where (m => m.IsPubliclyVisible && m.AccessFlags.HasFlag (MethodAccessFlags.Static)))
 				FixupExtensionMethod (method);

src/Xamarin.Android.Tools.Bytecode/Kotlin/KotlinUtilities.cs

@@ -72,6 +72,29 @@ public static string GetMethodNameWithoutSuffix (this MethodInfo method)
 			return index >= 0 ? method.Name.Substring (0, index) : method.Name;
 		}
 
+		public static bool IsDefaultConstructorMarker (this MethodInfo method)
+		{
+			// A default constructor is synthetic and always has an int and a
+			// DefaultConstructorMarker as its final 2 parameters.
+			if (method.Name != "<init>")
+				return false;
+
+			if (!method.AccessFlags.HasFlag (MethodAccessFlags.Synthetic))
+				return false;
+
+			var parameters = method.GetParameters ();
+
+			if (parameters.Length < 2)
+				return false;
+
+			// Final parameter is DefaultConstructorMarker
+			if (parameters.Last ().Type.TypeSignature != "Lkotlin/jvm/internal/DefaultConstructorMarker;")
+				return false;
+
+			// Next to final parameter is int
+			return parameters [parameters.Length - 2].Type.TypeSignature == "I";
+		}
+
 		public static bool IsPubliclyVisible (this ClassAccessFlags flags) => flags.HasFlag (ClassAccessFlags.Public) || flags.HasFlag (ClassAccessFlags.Protected);
 
 		public static bool IsPubliclyVisible (this KotlinClassVisibility flags) => flags == KotlinClassVisibility.Public || flags == KotlinClassVisibility.Protected;

tests/Xamarin.Android.Tools.Bytecode-Tests/KotlinFixupsTests.cs

@@ -51,6 +51,32 @@ public void HideInternalConstructor ()
 			Assert.False (ctor.AccessFlags.HasFlag (MethodAccessFlags.Public));
 		}
 
+		[Test]
+		public void HideDefaultConstructorMarker ()
+		{
+			var klass = LoadClassFile ("DefaultConstructor.class");
+
+			// init ()
+			var ctor_0p = klass.Methods.Single (m => m.Name == "<init>" && m.GetParameters ().Length == 0);
+
+			// init (string name)
+			var ctor_1p = klass.Methods.Single (m => m.Name == "<init>" && m.GetParameters ().Length == 1);
+
+			// init (string p0, int p1, DefaultConstructorMarker p2)
+			var ctor_3p = klass.Methods.Single (m => m.Name == "<init>" && m.GetParameters ().Length == 3);
+
+			Assert.True (ctor_3p.AccessFlags.HasFlag (MethodAccessFlags.Public));
+
+			KotlinFixups.Fixup (new [] { klass });
+
+			// Assert that the normal constructors are still public
+			Assert.True (ctor_0p.AccessFlags.HasFlag (MethodAccessFlags.Public));
+			Assert.True (ctor_1p.AccessFlags.HasFlag (MethodAccessFlags.Public));
+
+			// Assert that the synthetic "DefaultConstructorMarker" constructor has been marked private
+			Assert.False (ctor_3p.AccessFlags.HasFlag (MethodAccessFlags.Public));
+		}
+
 		[Test]
 		public void HideImplementationMethod ()
 		{

tests/Xamarin.Android.Tools.Bytecode-Tests/kotlin/DefaultConstructor.class

@@ -0,0 +1,2 @@
+class DefaultConstructor (name: String = "bob") {
+}

tests/Xamarin.Android.Tools.Bytecode-Tests/kotlin/DefaultConstructor.kt

@@ -9,8 +9,10 @@
       "src\\Java.Interop.Tools.Diagnostics\\Java.Interop.Tools.Diagnostics.csproj",
       "src\\Xamarin.Android.Tools.AnnotationSupport\\Xamarin.Android.Tools.AnnotationSupport.csproj",
       "src\\Xamarin.Android.Tools.ApiXmlAdjuster\\Xamarin.Android.Tools.ApiXmlAdjuster.csproj",
+      "src\\Xamarin.Android.Tools.Bytecode\\Xamarin.Android.Tools.Bytecode.csproj",
       "src\\Xamarin.SourceWriter\\Xamarin.SourceWriter.csproj",
       "tests\\generator-Tests\\generator-Tests.csproj",
+      "tests\\Xamarin.Android.Tools.Bytecode-Tests\\Xamarin.Android.Tools.Bytecode-Tests.csproj",
       "tests\\Xamarin.SourceWriter-Tests\\Xamarin.SourceWriter-Tests.csproj",
       "tools\\generator\\generator.csproj",
     ]