Restore C# version check in ICommandGenerator

CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/DiagnosticExtensions.cs

@@ -95,4 +95,24 @@ public static void ReportDiagnostics(this IncrementalGeneratorInitializationCont
             }
         });
     }
+
+    /// <summary>
+    /// Registers an output node into an <see cref="IncrementalGeneratorInitializationContext"/> to output diagnostics.
+    /// </summary>
+    /// <param name="context">The input <see cref="IncrementalGeneratorInitializationContext"/> instance.</param>
+    /// <param name="isSupported">An <see cref="IncrementalValueProvider{TValue}"/> instance indicating support for a feature.</param>
+    /// <param name="diagnostic">The <see cref="Diagnostic"/> to emit if <paramref name="isSupported"/> fails.</param>
+    public static void ReportDiagnosticsIsNotSupported(
+        this IncrementalGeneratorInitializationContext context,
+        IncrementalValueProvider<bool> isSupported,
+        Diagnostic diagnostic)
+    {
+        context.RegisterSourceOutput(isSupported, (context, isSupported) =>
+        {
+            if (!isSupported)
+            {
+                context.ReportDiagnostic(diagnostic);
+            }
+        });
+    }
 }

CommunityToolkit.Mvvm.SourceGenerators/Input/ICommandGenerator.cs

@@ -29,12 +29,23 @@ public sealed partial class ICommandGenerator : IIncrementalGenerator
     /// <inheritdoc/>
     public void Initialize(IncrementalGeneratorInitializationContext context)
     {
+        // Validate the language version
+        IncrementalValueProvider<bool> isGeneratorSupported =
+            context.ParseOptionsProvider
+            .Select(static (item, _) => item is CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 });
+
+        // Emit the diagnostic, if needed
+        context.ReportDiagnosticsIsNotSupported(isGeneratorSupported, Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+
         // Get all method declarations with at least one attribute
         IncrementalValuesProvider<IMethodSymbol> methodSymbols =
             context.SyntaxProvider
             .CreateSyntaxProvider(
                 static (node, _) => node is MethodDeclarationSyntax { Parent: ClassDeclarationSyntax, AttributeLists.Count: > 0 },
-                static (context, _) => (IMethodSymbol)context.SemanticModel.GetDeclaredSymbol(context.Node)!);
+                static (context, _) => (IMethodSymbol)context.SemanticModel.GetDeclaredSymbol(context.Node)!)
+            .Combine(isGeneratorSupported)
+            .Where(static item => item.Right)
+            .Select(static (item, _) => item.Left);
 
         // Filter the methods using [ICommand]
         IncrementalValuesProvider<(IMethodSymbol Symbol, AttributeData Attribute)> methodSymbolsWithAttributeData =