Disable Studio Add for record types the model cannot create

The window lost its disabled state when record creation moved into the model:
Add and Create were enabled for abstract types and threw on click, and
CreateElement(type, type) on an abstract [Union] type threw "X is not a choice
for X". CultInspectorModel.CanCreate is now the one rule CreateDefault and the
window both read, a slot's own type is always a choice, and an abstract one
returns the "abstract or no parameterless constructor" notice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/GameCult.Caching/CultInspectorModel.cs

@@ -345,14 +345,19 @@ namespace GameCult.Caching
             return new CultInspectorEdit(record, Clone(record.Document, record.Descriptor.DocumentType));
         }
 
+        // Whether CreateDefault makes a value: a string, struct, list or dictionary, or a concrete class with a parameterless constructor.
+        public bool CanCreate(Type type) =>
+            type == typeof(string) || type.IsValueType ||
+            ShapeOf(type).Kind is CultInspectorValueKind.List or CultInspectorValueKind.Dictionary ||
+            !type.IsAbstract && !type.IsInterface && type.GetConstructor(Type.EmptyTypes) != null;
+
         public object? CreateDefault(Type type)
         {
+            if (!CanCreate(type)) return null;
             if (type == typeof(string)) return string.Empty;
-            if (type.IsValueType) return Activator.CreateInstance(type);
             var shape = ShapeOf(type);
-            if (shape.Kind == CultInspectorValueKind.List && type.IsArray) return Array.CreateInstance(shape.ElementType!, 0);
-            if (shape.Kind == CultInspectorValueKind.Dictionary) return Activator.CreateInstance(shape.BuildType!);
-            return type.IsAbstract || type.IsInterface || type.GetConstructor(Type.EmptyTypes) == null ? null : Activator.CreateInstance(type);
+            if (type.IsArray) return Array.CreateInstance(shape.ElementType!, 0);
+            return Activator.CreateInstance(shape.Kind == CultInspectorValueKind.Dictionary ? shape.BuildType! : type);
         }
 
         // What a new value for a slot of elementType (a list element, a dictionary value, a union pick) may be: a union's
@@ -366,11 +371,10 @@ namespace GameCult.Caching
         // A new value of choice for a slot of elementType, or null with a notice when choice cannot be made.
         public object? CreateElement(Type elementType, Type choice, out string? notice)
         {
-            if (!ElementChoices(elementType).Contains(choice))
+            // A slot's own type is always offered; an abstract one is refused with the notice below.
+            if (choice != elementType && !ElementChoices(elementType).Contains(choice))
                 throw new ArgumentException($"{choice.Name} is not a choice for {elementType.Name}.", nameof(choice));
-            var created = choice == elementType ? CreateDefault(choice)
-                : choice.GetConstructor(Type.EmptyTypes) == null ? null
-                : Activator.CreateInstance(choice);
+            var created = CreateDefault(choice);
             // Null is a value only for a Nullable<T> slot; anywhere else it is nothing made.
             notice = created == null && !choice.IsValueType
                 ? $"{choice.Name} is abstract or has no parameterless constructor; nothing was created."

src/GameCult.Unity/Assets/Caching/Editor/CultCacheStudioWindow.cs

@@ -138,9 +138,10 @@ namespace GameCult.Unity.Caching.Editor
             {
                 var descriptor = _selectedType == null ? null : _cache.Registry.GetRequired(_selectedType);
                 var selected = Selected();
+                var cannotAdd = descriptor == null || ReadOnly || !_model.CanCreate(_selectedType);
                 using (new EditorGUILayout.HorizontalScope())
                 {
-                    using (new EditorGUI.DisabledScope(descriptor == null || descriptor.IsGlobal || ReadOnly))
+                    using (new EditorGUI.DisabledScope(cannotAdd || descriptor.IsGlobal))
                     {
                         if (GUILayout.Button("Add", EditorStyles.miniButtonLeft)) Add();
                     }
@@ -169,7 +170,7 @@ namespace GameCult.Unity.Caching.Editor
                         using (new EditorGUILayout.HorizontalScope())
                         {
                             EditorGUILayout.LabelField("Global is absent.");
-                            using (new EditorGUI.DisabledScope(ReadOnly))
+                            using (new EditorGUI.DisabledScope(cannotAdd))
                             {
                                 if (GUILayout.Button("Create", GUILayout.Width(56))) Add();
                             }

tests/GameCult.Caching.Tests/CultInspectorModelTests.cs

@@ -85,6 +85,11 @@ namespace GameCult.Caching.Tests
             Assert.That(bare, Does.Contain("parameterless"), "a plain element type that cannot be made says so; nothing is added");
             Assert.That(model.CreateElement(typeof(int?), typeof(int?), out var nullable), Is.Null);
             Assert.That(nullable, Is.Null, "null is a Nullable<T> element, not a failure");
+
+            Assert.That(model.CreateElement(typeof(InspectShape), typeof(InspectShape), out var @abstract), Is.Null);
+            Assert.That(@abstract, Does.Contain("abstract"), "a union's own type cannot be made; it says so instead of throwing");
+            Assert.That(new[] { typeof(InspectShape), typeof(InspectNeedsArgs), typeof(IList<int>) }.Any(model.CanCreate), Is.False);
+            Assert.That(new[] { typeof(InspectItem), typeof(string), typeof(int[]), typeof(IDictionary<string, int>), typeof(InspectOpenStruct) }.All(model.CanCreate), Is.True);
         }
 
         [Test]