[generator] Fixes WrapAttribute usage with interface protocols (#3885) (#4210)

* [generator] Fixes WrapAttribute usage with interface protocols

Fixes xamarin/xamarin-macios#3869

We need to support the scenario where `WrapAttribute` is used
to wrap an interface protocol type declaration like:

```csharp

	[Wrap ("WeakDelegate")]
	[NullAllowed]
	ICLSDataStoreDelegate Delegate { get; set; }

	[NullAllowed, Export ("delegate", ArgumentSemantic.Weak)]
	NSObject WeakDelegate { get; set; }

```

We generated invalid code for the setter that needed an explicit cast
and ended up with a CS0266 compiler error:

Bad generated code:

```csharp

[CompilerGenerated]
public ICLSDataStoreDelegate Delegate {
	get {
		return WeakDelegate as ICLSDataStoreDelegate;
	}
	set {
		WeakDelegate = value;
	}
}

```

Good generated code:

```csharp

[CompilerGenerated]
public ICLSDataStoreDelegate Delegate {
	get {
		return WeakDelegate as ICLSDataStoreDelegate;
	}
	set {
		var rvalue = value as NSObject;
		if (value != null && rvalue == null)
			throw new ArgumentException ("The object passed of type " + value.GetType () + " does not derive from NSObject");
		WeakDelegate = rvalue;
	}
}

```

src/generator.cs

@@ -4604,7 +4604,9 @@ public partial class Generator : IMemberGatherer {
 				print ("set {");
 				indent++;
 
-				if (minfo.protocolize){
+				var is_protocol_wrapper = UnifiedAPI && IsProtocolInterface (pi.PropertyType, false);
+
+				if (minfo.protocolize || is_protocol_wrapper){
 					print ("var rvalue = value as NSObject;");
 					print ("if (value != null && rvalue == null)");
 					print ("\tthrow new ArgumentException (\"The object passed of type \" + value.GetType () + \" does not derive from NSObject\");");
@@ -4616,7 +4618,7 @@ public partial class Generator : IMemberGatherer {
 					if (IsArrayOfWrappedType (pi.PropertyType))
 						print ("{0} = NSArray.FromNSObjects (value);", wrap);
 					else 
-						print ("{0} = {1}value;", wrap, minfo.protocolize ? "r" : "");
+						print ("{0} = {1}value;", wrap, minfo.protocolize || is_protocol_wrapper ? "r" : "");
 				}
 				indent--;
 				print ("}");

tests/generator/BGenTests.cs

@@ -525,6 +525,9 @@ namespace GeneratorTests
 		[Test]
 		public void Bug57870 () => BuildFile (Profile.iOS, true, true, "bug57870.cs");
 
+		[Test]
+		public void GHIssue3869 () => BuildFile (Profile.iOS, "ghissue3869.cs");
+
 		[Test]
 		[TestCase ("issue3875.cs", "AProtocol")]
 		[TestCase ("issue3875B.cs", "BProtocol")]

tests/generator/ghissue3869.cs

@@ -0,0 +1,26 @@
+using Foundation;
+using ObjCRuntime;
+
+namespace GHIssue3869 {
+	interface IFooDelegate { }
+
+	[Protocol, Model]
+	[BaseType (typeof (NSObject))]
+	interface FooDelegate {
+
+		[Abstract]
+		[Export ("doSomethingForIdentifier:parentContext:parentIdentifierPath:")]
+		NSObject DoSomething (string identifier, NSObject parentContext, string [] parentIdentifierPath);
+	}
+
+	[BaseType (typeof (NSObject))]
+	interface Foo {
+
+		[Wrap ("WeakDelegate")]
+		[NullAllowed]
+		IFooDelegate Delegate { get; set; }
+
+		[NullAllowed, Export ("delegate", ArgumentSemantic.Weak)]
+		NSObject WeakDelegate { get; set; }
+	}
+}