[generator] All bound interfaces should inherit IJavaPeerable (dotnet#455)

Context: dotnet#341
Context: dotnet#341 (comment)
Context: dotnet#25

Java 8 introduced support for [interface default methods][0]:

	// Java
	public interface Example {
	  int defaultInterfaceMethod() {
	    return 42;
	  }
	}

The question is, how should we bind them?

Currently, we *don't* bind interface default methods:

	// C#
	public interface IExample : IJavaObject {
	  // No `DefaultInterfaceMethod()` method
	}

This means that C# types which implement `IExample` don't need to
implement `Example.defaultInterfaceMethod()`, reducing the work that
needs to be done:

	// C#
	public class MyExample : Java.Lang.Object, IExample {
	  // No need to implement Example.defaultInterfaceMethod()!
	}

However, if a C# type *does* wish to implement
`Example.defaultInterfaceMethod()`, they *can* do so by using
[`ExportAttribute`][1], but this can be painful, as it requires
ensuring that the Java side and C# sides are consistent, without
compiler assistance:

	// C#
	partial class MyExample : Java.Lang.Object, IExample {
	  [Java.Interop.Export ("defaultInterfaceMethod")]
	  // If the above string is wrong, e.g. via typo, the method isn't overridden!
	  public int DefaultInterfaceMethod()
	  {
	    return 42*2;
	  }
	}

We want to improve support for this scenario by binding Java
interface default methods as [C#8 Default Interface Members][2],
as C#8 Default Interface Methods have similar semantics as Java 8
interface default methods, and means that `[ExportAttribute]` would
no longer be necessary to override them:

	// C#8?
	partial class MyExample : Java.Lang.Object, IExample {
	  // Just Works™, and the compiler will complain if there are typos.
	  public override int DefaultInterfaceMethod()
	  {
	    return 42*2;
	  }
	}

However, a question arises: how do we *do* that?

	// C#8
	public interface IExample : IJavaObject {
	  [Register ("defaultInterfaceMethod", "()I", ...)]
	  int DefaultInterfaceMethod()
	  {
	    // How do we call `Example.defaultInterfaceMethod()` here?
	  }
	}

The desirable thing would be for `IExample.DefaultInterfaceMethod()`
to have the same implementation as any other method binding, e.g.
when using `generator --codegen-target=XAJavaInterop1`:

	// C#8
	partial interface IExample {
	  [Register ("defaultInterfaceMethod", "()I", ...)]
	  void DefaultInterfaceMethod()
	  {
	    const string __id = "defaultInterfaceMethod.()I";
	    return _members.InstanceMethods.InvokeVirtualInt32Method (__id, this, null);
	  }
	}

The problem is twofold:

 1. There is no `_members` field to access here, and

 2. Even if there were a `_members` field,
    `JniPeerMembers.JniInstanceMethods.InvokeVirtualInt32Method()`
    requires an `IJavaPeerable` instance, and `IExample` doesn't
    implement `IJavaPeerable`.

(1) is straight forwardly solvable, as C#8 Default Interface Members
also adds support for static members of all sorts, so it should be
possible to add a static `_members` field, if deemed necessary.

(2) is the holdup, and has a simple solution: "Just" have every
bound interface *also* implement `IJavaPeerable`!

	- public partial interface IExample : IJavaObject
	+ public partial interface IExample : IJavaObject, IJavaPeerable

"But!", someone cries, "What about API compatibility?!"

Isn't adding `IJavaPeerable` to the implements list of an interface a
Breaking Change?

In theory and *most* practice, *Yes*, this *would* be a
Breaking Change, and thus something to be avoided.

*However*, Xamarin.Android is *not* "most" practice.

It has been an XA4212 error since ab3c2b2 / Xamarin.Android 8.0 to
implement an interface that implements `IJavaObject` *without*
inheriting from `Java.Lang.Object` or `Java.Lang.Throwable`:

	// elicits XA4212 error
	class MyBadClass : IExample {
	  // Implements IJavaObject.Handle
	  public IntPtr Handle {
	    get {return IntPtr.Zero;}
	  }
	}

Meanwhile, both `Java.Lang.Object` and `Java.Lang.Throwable` have
implemented `IJavaPeerable` since Xamarin.Android *6.1*.

*Because* it has been an error to manually implement `IJavaObject`
for nearly two years now, it should be Reasonably Safe™ to update
interfaces to implement *both* `IJavaObject` *and* `IJavaPeerable`.
Doing so should not break any code -- unless they've overridden the
`$(AndroidErrorOnCustomJavaObject)` MSBuild property to False, in
which case they deserve what happens to them.  (It's not really
possible to implement `IJavaObject` in a sane manner, and the
"straightforward" approach means that passing instances of e.g.
`MyBadClass` to Java code will result in passing *`null`* to Java,
which almost certainly is NOT what is intended, hence XA4212!)

[0]: https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
[1]: https://docs.microsoft.com/en-us/xamarin/android/platform/java-integration/working-with-jni#exportattribute-and-exportfieldattribute
[2]: https://github.com/dotnet/csharplang/blob/f7952cdddf85316a4beec493a0ecc14fcb3241c8/proposals/csharp-8.0/default-interface-methods.md

Author: jpobst

tools/generator/Java.Interop.Tools.Generator.CodeGeneration/CodeGenerator.cs

@@ -22,6 +22,8 @@ protected CodeGenerator (TextWriter writer, CodeGenerationOptions options)
 			opt = options;
 		}
 
+		internal virtual string GetAllInterfaceImplements () => "IJavaObject";
+
 		internal abstract void WriteClassHandle (ClassGen type, string indent, bool requireNew);
 
 		internal abstract void WriteClassHandle (InterfaceGen type, string indent, string declaringType);
@@ -522,7 +524,7 @@ public void WriteInterfaceDeclaration (InterfaceGen @interface, string indent)
 			if (@interface.TypeParameters != null && @interface.TypeParameters.Any ())
 				writer.WriteLine ("{0}{1}", indent, @interface.TypeParameters.ToGeneratedAttributeString ());
 			writer.WriteLine ("{0}{1} partial interface {2}{3} {{", indent, @interface.Visibility, @interface.Name,
-				@interface.IsConstSugar ? string.Empty : @interface.Interfaces.Count == 0 || sb.Length == 0 ? " : IJavaObject" : " : " + sb.ToString ());
+				@interface.IsConstSugar ? string.Empty : @interface.Interfaces.Count == 0 || sb.Length == 0 ? " : " + GetAllInterfaceImplements () : " : " + sb.ToString ());
 			WriteInterfaceFields (@interface, indent + "\t");
 			writer.WriteLine ();
 			WriteInterfaceProperties (@interface, indent + "\t");

tools/generator/Java.Interop.Tools.Generator.CodeGeneration/JavaInteropCodeGenerator.cs

@@ -23,6 +23,7 @@ static string GetInvokeType (string type)
 			}
 		}
 
+		internal override string GetAllInterfaceImplements () => "IJavaObject, IJavaPeerable";
 
 		internal override void WriteClassHandle (ClassGen type, string indent, bool requireNew)
 		{

tools/generator/Tests-Core/expected.ji/Android.Text.ISpanned.cs

@@ -7,7 +7,7 @@ namespace Android.Text {
 
 	// Metadata.xml XPath interface reference: path="/api/package[@name='android.text']/interface[@name='Spanned']"
 	[Register ("android/text/Spanned", "", "Android.Text.ISpannedInvoker")]
-	public partial interface ISpanned : IJavaObject {
+	public partial interface ISpanned : IJavaObject, IJavaPeerable {
 
 		// Metadata.xml XPath method reference: path="/api/package[@name='android.text']/interface[@name='Spanned']/method[@name='getSpanFlags' and count(parameter)=1 and parameter[1][@type='java.lang.Object']]"
 		[return:global::Android.Runtime.GeneratedEnum]

tools/generator/Tests-Core/expected.ji/Android.Views.View.cs

@@ -11,7 +11,7 @@ public partial class View : Java.Lang.Object {
 
 		// Metadata.xml XPath interface reference: path="/api/package[@name='android.view']/interface[@name='View.OnClickListener']"
 		[Register ("android/view/View$OnClickListener", "", "Android.Views.View/IOnClickListenerInvoker")]
-		public partial interface IOnClickListener : IJavaObject {
+		public partial interface IOnClickListener : IJavaObject, IJavaPeerable {
 
 			// Metadata.xml XPath method reference: path="/api/package[@name='android.view']/interface[@name='View.OnClickListener']/method[@name='onClick' and count(parameter)=1 and parameter[1][@type='android.view.View']]"
 			[Register ("onClick", "(Landroid/view/View;)V", "GetOnClick_Landroid_view_View_Handler:Android.Views.View/IOnClickListenerInvoker, ")]

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterface.txt

@@ -30,7 +30,7 @@ public abstract class MyInterfaceConsts : MyInterface {
 
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
-public partial interface IMyInterface : IJavaObject {
+public partial interface IMyInterface : IJavaObject, IJavaPeerable {
 
 	int Count {
 		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='get_Count' and count(parameter)=0]"

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterfaceDeclaration.txt

@@ -0,0 +1,39 @@
+// Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
+[Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
+public partial interface IMyInterface : IJavaObject, IJavaPeerable {
+
+	int Count {
+		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='get_Count' and count(parameter)=0]"
+		[Register ("get_Count", "()I", "Getget_CountHandler:java.code.IMyInterfaceInvoker, ")] get;
+		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='set_Count' and count(parameter)=1 and parameter[1][@type='int']]"
+		[Register ("set_Count", "(I)V", "Getset_Count_IHandler:java.code.IMyInterfaceInvoker, ")] set;
+	}
+
+	java.lang.String Key {
+		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='get_Key' and count(parameter)=0]"
+		[Register ("get_Key", "()Ljava/lang/String;", "Getget_KeyHandler:java.code.IMyInterfaceInvoker, ")] get;
+		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='set_Key' and count(parameter)=1 and parameter[1][@type='java.lang.String']]"
+		[Register ("set_Key", "(Ljava/lang/String;)V", "Getset_Key_Ljava_lang_String_Handler:java.code.IMyInterfaceInvoker, ")] set;
+	}
+
+	int AbstractCount {
+		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='get_AbstractCount' and count(parameter)=0]"
+		[Register ("get_AbstractCount", "()I", "Getget_AbstractCountHandler:java.code.IMyInterfaceInvoker, ")] get;
+		// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='set_AbstractCount' and count(parameter)=1 and parameter[1][@type='int']]"
+		[Register ("set_AbstractCount", "(I)V", "Getset_AbstractCount_IHandler:java.code.IMyInterfaceInvoker, ")] set;
+	}
+
+	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='GetCountForKey' and count(parameter)=1 and parameter[1][@type='java.lang.String']]"
+	[Register ("GetCountForKey", "(Ljava/lang/String;)I", "GetGetCountForKey_Ljava_lang_String_Handler:java.code.IMyInterfaceInvoker, ")]
+	int GetCountForKey (string key);
+
+	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='Key' and count(parameter)=0]"
+	[Register ("Key", "()Ljava/lang/String;", "GetKeyHandler:java.code.IMyInterfaceInvoker, ")]
+	string Key ();
+
+	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='AbstractMethod' and count(parameter)=0]"
+	[Register ("AbstractMethod", "()V", "GetAbstractMethodHandler:java.code.IMyInterfaceInvoker, ")]
+	void AbstractMethod ();
+
+}
+

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterfaceFields.txt

@@ -1,6 +1,6 @@
 // Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
 [Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
-public partial interface IMyInterface : IJavaObject {
+public partial interface IMyInterface : IJavaObject, IJavaPeerable {
 
 	// Metadata.xml XPath field reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/field[@name='MyConstantField']"
 	[Register ("MyConstantField")]

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/JavaInterop1/WriteInterfaceMethod.txt

@@ -0,0 +1,11 @@
+// Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
+[Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
+public partial interface IMyInterface : IJavaObject, IJavaPeerable {
+
+	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='DoSomething' and count(parameter)=0]"
+	[global::Java.Interop.JavaInterfaceDefaultMethod]
+	[Register ("DoSomething", "()V", "GetDoSomethingHandler:java.code.IMyInterfaceInvoker, ")]
+	void DoSomething ();
+
+}
+

tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/Common/WriteInterfaceDeclaration.txt renamed to tools/generator/Tests/Unit-Tests/CodeGeneratorExpectedResults/XamarinAndroid/WriteInterfaceDeclaration.txt

@@ -0,0 +1,11 @@
+// Metadata.xml XPath interface reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']"
+[Register ("java/code/IMyInterface", "", "java.code.IMyInterfaceInvoker")]
+public partial interface IMyInterface : IJavaObject {
+
+	// Metadata.xml XPath method reference: path="/api/package[@name='java.code']/interface[@name='IMyInterface']/method[@name='DoSomething' and count(parameter)=0]"
+	[global::Java.Interop.JavaInterfaceDefaultMethod]
+	[Register ("DoSomething", "()V", "GetDoSomethingHandler:java.code.IMyInterfaceInvoker, ")]
+	void DoSomething ();
+
+}
+