C#: Fix to allow usage of [MustBeVariant] in generic typed attributes

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/MustBeVariantAnalyzer.cs

@@ -109,11 +109,19 @@ namespace Godot.SourceGenerators
         /// <returns><see langword="true"/> if the type must be variant and must be analyzed.</returns>
         private bool ShouldCheckTypeArgument(SyntaxNodeAnalysisContext context, SyntaxNode parentSyntax, ISymbol parentSymbol, TypeSyntax typeArgumentSyntax, ITypeSymbol typeArgumentSymbol, int typeArgumentIndex)
         {
-            var typeParamSymbol = parentSymbol switch
+            ITypeParameterSymbol? typeParamSymbol = parentSymbol switch
             {
-                IMethodSymbol methodSymbol => methodSymbol.TypeParameters[typeArgumentIndex],
-                INamedTypeSymbol typeSymbol => typeSymbol.TypeParameters[typeArgumentIndex],
-                _ => null,
+                IMethodSymbol methodSymbol when parentSyntax.Parent is AttributeSyntax &&
+                                                methodSymbol.ContainingType.TypeParameters.Length > 0
+                    => methodSymbol.ContainingType.TypeParameters[typeArgumentIndex],
+
+                IMethodSymbol { TypeParameters.Length: > 0 } methodSymbol
+                    => methodSymbol.TypeParameters[typeArgumentIndex],
+
+                INamedTypeSymbol { TypeParameters.Length: > 0 } typeSymbol
+                    => typeSymbol.TypeParameters[typeArgumentIndex],
+                _
+                    => null
             };
 
             if (typeParamSymbol == null)