async void does not allow

Author: neuecc

src/ConsoleAppFramework/DiagnosticDescriptors.cs

@@ -118,4 +118,8 @@ public static DiagnosticDescriptor Create(int id, string title, string messageFo
     public static DiagnosticDescriptor DocCommentParameterNameNotMatched { get; } = Create(
         15,
         "Document Comment parameter name '{0}' does not match method parameter name.");
+
+    public static DiagnosticDescriptor ReturnTypeMethodAsyncVoid { get; } = Create(
+        16,
+        "Command method return type does not allow async void.");
 }

src/ConsoleAppFramework/Parser.cs

@@ -414,6 +414,18 @@ internal class Parser(ConsoleAppFrameworkGeneratorOptions generatorOptions, Diag
         if (methodSymbol.ReturnType.SpecialType == SpecialType.System_Void)
         {
             isVoid = true;
+
+            // check `async void`
+            var syntax = methodSymbol.DeclaringSyntaxReferences[0].GetSyntax() as MethodDeclarationSyntax;
+            if (syntax != null)
+            {
+                var asyncKeyword = syntax.Modifiers.FirstOrDefault(x => x.IsKind(SyntaxKind.AsyncKeyword));
+                if (asyncKeyword != default)
+                {
+                    context.ReportDiagnostic(DiagnosticDescriptors.ReturnTypeMethodAsyncVoid, asyncKeyword.GetLocation());
+                    return null;
+                }
+            }
         }
         else if (methodSymbol.ReturnType.SpecialType == SpecialType.System_Int32)
         {

tests/ConsoleAppFramework.GeneratorTests/DiagnosticsTest.cs

@@ -456,5 +456,24 @@ public void Bar(string msg)
 """, "Bar");
 
     }
+
+    [Fact]
+    public void AsyncVoid()
+    {
+        verifier.Verify(16, """
+var app = ConsoleApp.Create();
+app.Add<MyCommands2>();
+app.Run(args);
+
+public class MyCommands2
+{
+    public async void Foo()
+    {
+        await Task.Yield();
+    }
+}
+
+""", "async");
+    }
 }