[generator] Don't invalidate interface if static method is invalidated (#660)

Fixes: #588

If we cannot bind a method on an interface we mark the interface as
unbindable and omit the interface from the generated code, and
invalidate anything that depends on the interface.  This is because
an interface with a missing method cannot be implemented in C#.

However, if the method is `static` then we *can* remove the method
while keeping the interface, as the user is not required to implement
the static method.

Update `GenBase.OnValidate()` so that invalid static methods don't
invalidate their declaring interface.

Note we already properly handled Default Interface Methods, this
adds support for `static` methods as well.

Author: jpobst

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

@@ -396,5 +396,40 @@ public void RespectNoAlternativesForInterfaces ()
 			Assert.False (writer.ToString ().Contains ("class ParentConsts"));
 			Assert.False (writer.ToString ().Contains ("class Parent"));
 		}
+
+		[Test]
+		public void DontInvalidateInterfaceDueToStaticOrDefaultMethods ()
+		{
+			// This interface contains a static and a default interface method that cannot
+			// be bound due to an unknown return type. However the user doesn't have to
+			// provide an implementation for these methods, so it's ok to bind the interface.
+
+			var xml = @"<api>
+			  <package name='com.xamarin.android' jni-name='com/xamarin/android'>
+			    <interface abstract='true' deprecated='not deprecated' final='false' name='Parent' static='false' visibility='public' jni-signature='Lcom/xamarin/android/Parent;'>
+			      <method abstract='true' deprecated='not deprecated' name='normalMethod' jni-signature='()I' return='int' static='false' transient='false' visibility='public' volatile='false'></method>
+			      <method abstract='false' deprecated='not deprecated' name='staticMethod' jni-signature='()Lfoo/bar/baz;' return='Lfoo/bar/baz;' static='true' transient='false' visibility='public' volatile='false'></method>
+			      <method abstract='false' deprecated='not deprecated' name='defaultMethod' jni-signature='()Lfoo/bar/baz;' return='Lfoo/bar/baz;' static='false' transient='false' visibility='public' volatile='false'></method>
+			    </interface>
+			  </package>
+			</api>";
+
+			var gens = ParseApiDefinition (xml);
+			var iface = gens.OfType<InterfaceGen> ().Single ();
+
+			var result = iface.Validate (options, new GenericParameterDefinitionList (), new CodeGeneratorContext ());
+
+			// Inteface should pass validation despite invalid static/default methods
+			Assert.True (result);
+
+			generator.WriteInterface (iface, string.Empty, new GenerationInfo (string.Empty, string.Empty, "MyAssembly"));
+
+			var generated = writer.ToString ();
+
+			Assert.True (generated.Contains ("interface IParent"));
+			Assert.True (generated.Contains ("NormalMethod"));
+			Assert.False (generated.Contains ("StaticMethod"));
+			Assert.False (generated.Contains ("DefaultMethod"));
+		}
 	}
 }

tools/generator/Java.Interop.Tools.Generator.ObjectModel/GenBase.cs

@@ -677,11 +677,11 @@ protected virtual bool OnValidate (CodeGenerationOptions opt, GenericParameterDe
 			}
 			Fields = valid_fields;
 
-			// If we can't validate a default interface method it's ok to ignore it and still bind the interface
-			var method_cnt = Methods.Where (m => !m.IsInterfaceDefaultMethod).Count ();
+			// If we can't validate a static or default interface method it's ok to ignore it and still bind the interface
+			var method_cnt = Methods.Where (m => !m.IsInterfaceDefaultMethod && !m.IsStatic).Count ();
 
 			Methods = Methods.Where (m => ValidateMethod (opt, m, context)).ToList ();
-			MethodValidationFailed = method_cnt != Methods.Where (m => !m.IsInterfaceDefaultMethod).Count ();
+			MethodValidationFailed = method_cnt != Methods.Where (m => !m.IsInterfaceDefaultMethod && !m.IsStatic).Count ();
 
 			foreach (Method m in Methods) {
 				if (m.IsVirtual)