[generator] Fix generation of ReturnRelease code for third-party bindings. (#6135)

Don't use internal platform API (the Selector.Release field), and make sure
the void_objc_msgSend function exists in the binding.

src/generator.cs

@@ -4449,14 +4449,23 @@ public partial class Generator : IMemberGatherer {
 		}
 		
 		if (minfo.is_return_release) {
+
+			// Make sure we generate the required signature in Messaging only if needed 
+			// bool_objc_msgSendSuper_IntPtr: for respondsToSelector:
+			if (!send_methods.ContainsKey ("void_objc_msgSend")) {
+				print (m, "[DllImport (LIBOBJC_DYLIB, EntryPoint=\"objc_msgSendSuper\")]");
+				print (m, "public extern static void void_objc_msgSend (IntPtr receiever, IntPtr selector);");
+				RegisterMethodName ("void_objc_msgSend");
+			}
+
 			if (!needsPtrZeroCheck)
-				print ("{0}.void_objc_msgSend (ret.Handle, Selector.GetHandle (Selector.Release));", ns.Messaging);
+				print ("global::{0}.void_objc_msgSend (ret.Handle, Selector.GetHandle (\"release\"));", ns.Messaging);
 			else {
 				// We must create the managed wrapper before calling Release on it
 				// FIXME: https://trello.com/c/1ukS9TbL/43-introduce-common-object-type-for-all-unmanaged-types-which-will-correctly-implement-idisposable-and-inativeobject
 				// We should consider using return INativeObject<T> (ptr, bool); here at some point
 				print ("global::{0} relObj = ret == IntPtr.Zero ? null : new global::{0} (ret);", mi.ReturnType.FullName);
-				print ("if (relObj != null) global::{0}.void_objc_msgSend (relObj.Handle, Selector.GetHandle (Selector.Release));", ns.Messaging);
+				print ("if (relObj != null) global::{0}.void_objc_msgSend (relObj.Handle, Selector.GetHandle (\"release\"));", ns.Messaging);
 			}
 		}
 		

tests/generator/BGenTests.cs

@@ -588,10 +588,16 @@ namespace GeneratorTests
 		[Test]
 		public void GHIssue5692 () => BuildFile (Profile.iOS, "ghissue5692.cs");
 
+		[Test]
 		public void RefOutParameters ()
 		{
 			BuildFile (Profile.macOSMobile, true, "tests/ref-out-parameters.cs");
+		}
 
+		[Test]
+		public void ReturnRelease ()
+		{
+			BuildFile (Profile.iOS, "tests/return-release.cs");
 		}
 
 		BGenTool BuildFile (Profile profile, params string [] filenames)

tests/generator/generator-tests.csproj

@@ -70,7 +70,8 @@
   <ItemGroup>
     <None Include="tests\is-direct-binding.cs" />
     <None Include="packages.config" />
-    <None Include="tests\ref-out-parameters.cs" />
+    <None Include="tests\ref-out-parameters.cs" />
+    <None Include="tests\return-release.cs" />
   </ItemGroup>
   <ItemGroup>
     <Folder Include="tests\" />

tests/generator/tests/return-release.cs

@@ -0,0 +1,18 @@
+using System;
+using Foundation;
+
+namespace NS
+{
+	[Protocol]
+	interface ProtocolWithReturnRelease
+	{
+		[Abstract, Export ("newRequiredObject")]
+		[return: Release]
+		NSObject CreateRequiredObject ();
+
+		[Export ("newOptionalObject")]
+		[return: Release]
+		NSObject CreateOptionalObject ();
+	}
+}
+