[generator] Fix CS0115 error "overriding" default interface method (#489)

Given this Java hierarchy:

	// Java
	public interface DefaultMethodsInterface
	{
	    default int getBar () { return 2; }
	}
	
	public class EmptyOverrideClass implements DefaultMethodsInterface
	{
	}
	
	public class ImplementedChainOverrideClass extends EmptyOverrideClass
	{
	    @OverRide
	    public int getBar () { return 100; }
	}

We were improperly binding `ImplementedChainOverrideClass.Bar` as an
`override`:

	// C# binding
	public partial interface IDefaultMethodsInterface : IJavaPeerable {
	    virtual unsafe int Bar {
	        [Register ("getBar", "()I", "...")]
	        get { /* ... */}
	    }
	}

	public partial class EmptyOverrideClass : Java.Lang.Object, IDefaultMethodsInterface {
	}

	public partial class ImplementedChainOverrideClass : EmptyOverrideClass {
	    public override unsafe int Bar { /* ... */ }
	}

This results in a CS0115:

	CS0115: 'ImplementedChainOverrideClass.Bar': no suitable method found to override

C# doesn't allow using the `override` keyword with default interface
methods; see also [dotnet/csharplang@2757][0].

We need to instead emit `virtual` in this context:

	public partial class ImplementedChainOverrideClass : EmptyOverrideClass {
	    public virtual unsafe int Bar { /* ... */ }
	}

[0]: dotnet/csharplang#2757

Author: jpobst

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

@@ -1470,8 +1470,13 @@ public void WriteProperty (Property property, GenBase gen, string indent, bool w
 			if ((property.Getter ?? property.Setter).IsStatic)
 				virtual_override = " static";
 			// It should be using AdjustedName instead of Name, but ICharSequence ("Formatted") properties are not caught by this...
-			else if (gen.BaseSymbol != null && gen.BaseSymbol.GetPropertyByName (property.Name, true) != null)
-				virtual_override = " override";
+			else if (gen.BaseSymbol != null) {
+				var base_prop = gen.BaseSymbol.GetPropertyByName (property.Name, true);
+
+				// If the matching base getter we found is a DIM, we do not override it, it should stay virtual
+				if (base_prop != null && !base_prop.Getter.IsInterfaceDefaultMethod)
+					virtual_override = " override";
+			}
 
 			WriteMethodIdField (property.Getter, indent);
 			if (property.Setter != null)

tools/generator/Tests/Unit-Tests/CodeGeneratorTestBase.cs

@@ -1,7 +1,10 @@
 using System;
+using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Reflection;
 using System.Text;
+using System.Xml.Linq;
 using MonoDroid.Generation;
 using NUnit.Framework;
 
@@ -55,5 +58,25 @@ protected string GetTargetedExpected (string testName)
 
 			return File.ReadAllText (Path.Combine (root, "Unit-Tests", "CodeGeneratorExpectedResults", target, $"{testName}.txt")).NormalizeLineEndings ();
 		}
+
+		protected List<GenBase> ParseApiDefinition (string xml)
+		{
+			var doc = XDocument.Parse (xml);
+			var gens = new Parser (options).Parse (doc, Enumerable.Empty<string> (), "29", 7);
+
+			foreach (var gen in gens)
+				options.SymbolTable.AddType (gen);
+
+			foreach (var gen in gens)
+				gen.Validate (options, new GenericParameterDefinitionList (), generator.Context);
+
+			foreach (var gen in gens)
+				gen.FillProperties ();
+
+			foreach (var gen in gens)
+				gen.FixupMethodOverrides (options);
+
+			return gens;
+		}
 	}
 }

tools/generator/Tests/Unit-Tests/DefaultInterfaceMethodsTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using MonoDroid.Generation;
 using NUnit.Framework;
 using Xamarin.Android.Binder;
@@ -149,6 +150,48 @@ public void WriteSealedOverriddenDefaultMethod ()
 			Assert.False (writer.ToString ().Contains ("virtual sealed"));
 		}
 
+		[Test]
+		public void WriteInterfaceRedeclaredChainDefaultMethod ()
+		{
+			// Fix a case where a property declared in this hierarchy was generated as "override" instead of "virtual"
+			// public interface MyInterface { default int getValue () { return 0; } }
+			// public class MyClass implements MyInterface { }
+			// public class MySecondClass extends MyClass { @Override public int getValue () { return 1; } }
+			var gens = ParseApiDefinition (@"<api>
+			  <package name='java.lang' jni-name='java/lang'>
+			    <class abstract='false' deprecated='not deprecated' final='false' name='Object' static='false' visibility='public' jni-signature='Ljava/lang/EmptyOverrideClass;' />
+			  </package>
+			  <package name='com.xamarin.android' jni-name='com/xamarin/android'>
+			    <interface abstract='true' deprecated='not deprecated' final='false' name='DefaultMethodsInterface' static='false' visibility='public' jni-signature='Lcom/xamarin/android/DefaultMethodsInterface;'>
+			      <method abstract='false' deprecated='not deprecated' final='false' name='foo' jni-signature='()I' bridge='false' native='false' return='int' jni-return='I' static='false' synchronized='false' synthetic='false' visibility='public'></method>
+			      <method abstract='false' deprecated='not deprecated' final='false' name='getBar' jni-signature='()I' bridge='false' native='false' return='int' jni-return='I' static='false' synchronized='false' synthetic='false' visibility='public'></method>
+			      <method abstract='false' deprecated='not deprecated' final='false' name='setBar' jni-signature='(I)V' bridge='false' native='false' return='void' jni-return='V' static='false' synchronized='false' synthetic='false' visibility='public'>
+				<parameter name='p0' type='int' jni-type='I'></parameter>
+			      </method>
+			    </interface>
+			    <class abstract='false' deprecated='not deprecated' extends='java.lang.Object' extends-generic-aware='java.lang.Object' jni-extends='Ljava/lang/Object;' final='false' name='EmptyOverrideClass' static='false' visibility='public' jni-signature='Lcom/xamarin/android/EmptyOverrideClass;'>
+			      <implements name='com.xamarin.android.DefaultMethodsInterface' name-generic-aware='com.xamarin.android.DefaultMethodsInterface' jni-type='Lcom/xamarin/android/DefaultMethodsInterface;'></implements>
+			      <constructor deprecated='not deprecated' final='false' name='EmptyOverrideClass' jni-signature='()V' bridge='false' static='false' type='com.xamarin.android.EmptyOverrideClass' synthetic='false' visibility='public'></constructor>
+			    </class>
+			    <class abstract='false' deprecated='not deprecated' extends='com.xamarin.android.EmptyOverrideClass' extends-generic-aware='com.xamarin.android.EmptyOverrideClass' jni-extends='Lcom/xamarin/android/EmptyOverrideClass;' final='false' name='ImplementedChainOverrideClass' static='false' visibility='public' jni-signature='Lcom/xamarin/android/ImplementedChainOverrideClass;'>
+			      <constructor deprecated='not deprecated' final='false' name='ImplementedChainOverrideClass' jni-signature='()V' bridge='false' static='false' type='com.xamarin.android.ImplementedChainOverrideClass' synthetic='false' visibility='public'></constructor>
+			      <method abstract='false' deprecated='not deprecated' final='false' name='getBar' jni-signature='()I' bridge='false' native='false' return='int' jni-return='I' static='false' synchronized='false' synthetic='false' visibility='public'></method>
+			      <method abstract='false' deprecated='not deprecated' final='false' name='setBar' jni-signature='(I)V' bridge='false' native='false' return='void' jni-return='V' static='false' synchronized='false' synthetic='false' visibility='public'>
+				<parameter name='p0' type='int' jni-type='I'></parameter>
+			      </method>
+			    </class>
+			  </package>
+			</api>");
+
+			var klass = (ClassGen)gens.First (g => g.Name == "ImplementedChainOverrideClass");
+
+			generator.Context.ContextTypes.Push (klass);
+			generator.WriteClass (klass, string.Empty, new GenerationInfo (string.Empty, string.Empty, "MyAssembly"));
+			generator.Context.ContextTypes.Pop ();
+
+			Assert.True (writer.ToString ().Contains ("public virtual unsafe int Bar"));
+		}
+
 		[Test]
 		public void WriteStaticInterfaceMethod ()
 		{