[RGen] Raise a warning in an async method that will use a nameless tuple.

src/rgen/Microsoft.Macios.Bindings.Analyzer/Resources.resx

@@ -534,4 +534,17 @@
         <value>Duplicated async name</value>
     </data>
 
+    <!-- RBI0040 -->
+
+    <data name="RBI0040Description" xml:space="preserve">
+        <value>No result type or result type name was provided for an async method.</value>
+    </data>
+    <data name="RBI0040MessageFormat" xml:space="preserve">
+            <value>The method '{0}' was marked as async and has multiple parameters but does not provide a return type name, a nameless tuple will be generated for the async method</value>
+        <comment>{0} is the name of the async method.</comment>
+    </data>
+    <data name="RBI0040Title" xml:space="preserve">
+        <value>Not specified return type</value>
+    </data>
+
 </root>

src/rgen/Microsoft.Macios.Bindings.Analyzer/RgenDiagnostics.cs

@@ -601,4 +601,19 @@ public static class RgenDiagnostics {
 		description: new LocalizableResourceString (nameof (Resources.RBI0039Description), Resources.ResourceManager,
 			typeof (Resources))
 	);
+
+	/// <summary>
+	/// Disgnostic descriptor for when a method marked as async has does not provide a return type or return type name.
+	/// </summary>
+	internal static readonly DiagnosticDescriptor RBI0040 = new (
+		"RBI0040",
+		new LocalizableResourceString (nameof (Resources.RBI0040Title), Resources.ResourceManager, typeof (Resources)),
+		new LocalizableResourceString (nameof (Resources.RBI0040MessageFormat), Resources.ResourceManager,
+			typeof (Resources)),
+		"Usage",
+		DiagnosticSeverity.Warning,
+		isEnabledByDefault: true,
+		description: new LocalizableResourceString (nameof (Resources.RBI0040Description), Resources.ResourceManager,
+			typeof (Resources))
+	);
 }

src/rgen/Microsoft.Macios.Bindings.Analyzer/Validators/ClassValidator.cs

@@ -259,6 +259,23 @@ bool ValidAsyncMethods (Binding binding, RootContext context,
 						// add a new list with the current property
 						asyncMethodNames.Add (asyncMethodKey, [(currentMethod.Name, currentMethod.Location)]);
 					}
+
+					// add a extra validation, if current method has more than one parameter + the delegate and does 
+					// not provide a return type, create a warning for the user to avoid the usage of a nameless
+					// tuple
+					if (currentMethod.Parameters.Length > 2 // more than one parameter + the delegate
+						&& currentMethod.ExportMethodData.ResultType.IsNullOrDefault // does not provide a return type
+						&& currentMethod.ExportMethodData.ResultTypeName is null // does not provide a return type name to generate
+						) {
+						// create warning to inform that a nameless tuple will be generated for the async method
+						// and that it is better to provide a return type name
+						builder.Add (Diagnostic.Create (
+							descriptor: RBI0040, // The method '{0}' was marked as async and has multiple parameters but does not provide a return type name, a nameless tuple will be generated for the async method
+							location: currentMethod.Location,
+							messageArgs: [
+								currentMethod.Name,
+							]));
+					}
 				}
 			} else {
 				// this is not an async method, but we need to check if it has a callback delegate parameter,
@@ -315,7 +332,7 @@ public ClassValidator ()
 
 		// validate async methods. This is a global strategy because it needs to look at all the methods in the binding
 		// are validated together so that async methods do not have the same names
-		AddGlobalStrategy ([RBI0035, RBI0036, RBI0037, RBI0038, RBI0039], ValidAsyncMethods);
+		AddGlobalStrategy ([RBI0035, RBI0036, RBI0037, RBI0038, RBI0039, RBI0040], ValidAsyncMethods);
 
 		// validate that strong delegates are not duplicated, this is only for weak properties
 		AddStrategy (

tests/rgen/Microsoft.Macios.Bindings.Analyzer.Tests/ClassAnalyzerTests.cs

@@ -818,6 +818,44 @@ public partial class TestClass{
 				DiagnosticSeverity.Warning,
 				"The method 'GetCount' was not marked as async but it can be"
 			];
+
+			// correct async method but we are missing the return type
+			yield return [
+				@"
+#pragma warning disable APL0003
+
+using System;
+using System.Runtime.Versioning;
+using AVFoundation;
+using CoreGraphics;
+using Foundation;
+using ObjCBindings;
+using ObjCRuntime;
+using nfloat = System.Runtime.InteropServices.NFloat;
+
+namespace TestNamespace;
+
+[SupportedOSPlatform (""macos"")]
+[SupportedOSPlatform (""ios"")]
+[SupportedOSPlatform (""tvos"")]
+[SupportedOSPlatform (""maccatalyst13.1"")]
+[BindingType<Class>]
+public partial class TestClass{
+
+	[SupportedOSPlatform (""ios"")]
+	[SupportedOSPlatform (""tvos"")]
+	[SupportedOSPlatform (""macos"")]
+	[SupportedOSPlatform (""maccatalyst13.1"")]
+	[Export<Method> (""count:"", 
+		Flags = ObjCBindings.Method.Async,
+		MethodName = ""GetCountAsync"")]
+	public virtual partial void GetCount (int first, int second, Action callback);
+
+}",
+				"RBI0040",
+				DiagnosticSeverity.Warning,
+				"The method 'GetCount' was marked as async and has multiple parameters but does not provide a return type name, a nameless tuple will be generated for the async method"
+			];
 		}
 
 		IEnumerator IEnumerable.GetEnumerator () => GetEnumerator ();
@@ -1044,6 +1082,80 @@ public partial class TestClass{
 }",
 				"RBI0039",
 			];
+
+			// correct async method but with return type
+			yield return [
+				@"
+#pragma warning disable APL0003
+
+using System;
+using System.Runtime.Versioning;
+using AVFoundation;
+using CoreGraphics;
+using Foundation;
+using ObjCBindings;
+using ObjCRuntime;
+using nfloat = System.Runtime.InteropServices.NFloat;
+
+namespace TestNamespace;
+
+[SupportedOSPlatform (""macos"")]
+[SupportedOSPlatform (""ios"")]
+[SupportedOSPlatform (""tvos"")]
+[SupportedOSPlatform (""maccatalyst13.1"")]
+[BindingType<Class>]
+public partial class TestClass{
+
+	[SupportedOSPlatform (""ios"")]
+	[SupportedOSPlatform (""tvos"")]
+	[SupportedOSPlatform (""macos"")]
+	[SupportedOSPlatform (""maccatalyst13.1"")]
+	[Export<Method> (""count:"", 
+		Flags = ObjCBindings.Method.Async,
+		ResultType = typeof ((int First, int Second)),
+		MethodName = ""GetCountAsync"")]
+	public virtual partial void GetCount (int first, int second, Action callback);
+
+}",
+				"RBI0040",
+			];
+
+			// correct async method but with return type name
+			yield return [
+				@"
+#pragma warning disable APL0003
+
+using System;
+using System.Runtime.Versioning;
+using AVFoundation;
+using CoreGraphics;
+using Foundation;
+using ObjCBindings;
+using ObjCRuntime;
+using nfloat = System.Runtime.InteropServices.NFloat;
+
+namespace TestNamespace;
+
+[SupportedOSPlatform (""macos"")]
+[SupportedOSPlatform (""ios"")]
+[SupportedOSPlatform (""tvos"")]
+[SupportedOSPlatform (""maccatalyst13.1"")]
+[BindingType<Class>]
+public partial class TestClass{
+
+	[SupportedOSPlatform (""ios"")]
+	[SupportedOSPlatform (""tvos"")]
+	[SupportedOSPlatform (""macos"")]
+	[SupportedOSPlatform (""maccatalyst13.1"")]
+	[Export<Method> (""count:"", 
+		Flags = ObjCBindings.Method.Async,
+		ResultTypeName = ""AsyncResult""
+		MethodName = ""GetCountAsync"")]
+	public virtual partial void GetCount (int first, int second, Action callback);
+
+}",
+				"RBI0040",
+			];
 		}
 
 		IEnumerator IEnumerable.GetEnumerator () => GetEnumerator ();