Still needs implementation for RNA, declarations, and modifier
integration.

The socket stores an int identifier as a default input value, as well as
a weak reference to the enum definition (node tree pointer + node
identifier). Identifiers for enum values are unique and only used once,
so that library-linked files can be changed without breaking enum values
or creating ambiguous identifiers.

The reference will be updated by the NodeTreeMainUpdater
when node connections change. Enum references propagate right-to-left,
taking care to verify that all connections use the same enum or the
reference becomes invalid.

The reference can be resolved at runtime to get the full list of enum
values. This will be wrapped in a RNA enum as well. The IDProperty on
the modifier side will probably be a simple string, at least for now.

@@ -103,4 +103,17 @@ class StringBakeItem : public BakeItem {
   }
 };
 
+class EnumBakeItem : public BakeItem {

@@ -993,2 +996,4 @@
           break;
         }
+        case SOCK_ENUM:
+          uiItemL(sub, "ENUM SOCKET TODO", ICON_NONE);

@@ -315,3 +330,4 @@
       return;
     }
 
+    /* XXX This isn't great yet, it's fast but could be avoided if

@@ -372,1 +392,3 @@
-            }
+          if (md->type == eModifierType_Nodes) {
+            MOD_nodes_update_interface(
+                object, (NodesModifierData *)md, /*recalc=*/result.interface_changed);

@@ -781,0 +861,4 @@
+
+    for (const bNodeSocket *src : src_span) {
+      if (src->is_available() && src->type == SOCK_MENU) {
+        update_socket_enum_definition(*dst.default_value_typed<bNodeSocketValueMenu>(),

@@ -1199,6 +1199,7 @@ static const float std_node_socket_colors[][4] = {
     {0.62, 0.31, 0.64, 1.0}, /* SOCK_TEXTURE */
     {0.92, 0.46, 0.51, 1.0}, /* SOCK_MATERIAL */
     {0.65, 0.39, 0.78, 1.0}, /* SOCK_ROTATION */
+    {1.00, 1.00, 1.00, 1.0}, /* SOCK_ROTATION */

@@ -0,0 +85,4 @@
+    case SOCK_MENU:
+      /* Technically possible perhaps to select an enum based on
+       * another enum, but not supported for now. */
+      BLI_assert_unreachable();

@@ -891,6 +988,11 @@ void update_output_properties_from_node_tree(const bNodeTree &tree,
     const bNodeSocketType *typeinfo = socket.socket_typeinfo();
     const eNodeSocketDatatype socket_type = typeinfo ? eNodeSocketDatatype(typeinfo->type) :
                                                        SOCK_CUSTOM;
+    /* Enum sockets get updated every time, because enum item changes do not trigger a full

@@ -486,6 +490,7 @@ class NodeTreeMainUpdater {
     this->make_node_previews_dirty(ntree);
 
     this->propagate_runtime_flags(ntree);
+    this->propagate_enum_definitions(ntree);

@@ -801,6 +806,186 @@ class NodeTreeMainUpdater {
     }
   }
 
+  void reset_enum_ptr(bNodeSocketValueMenu &dst)

@@ -20,3 +20,3 @@
  * the values.
  */
-void MOD_nodes_update_interface(Object *object, NodesModifierData *nmd);
+void MOD_nodes_update_properties(NodesModifierData *nmd);

@@ -0,0 +16,4 @@
+/**
+ * Runtime copy of #NodeEnumItem for use in #RuntimeNodeEnumItems.
+ */
+struct RuntimeNodeEnumItem {

@@ -73,6 +73,8 @@ PrimitiveBakeItem::~PrimitiveBakeItem()
 
 StringBakeItem::StringBakeItem(std::string value) : value_(std::move(value)) {}
 
+EnumBakeItem::EnumBakeItem(const int32_t value) : value_(std::move(value)) {}

@@ -922,0 +935,4 @@
+} bNodeSocketValueMenu;
+
+/* Flags for #bNodeSocketValueMenu. */
+typedef enum NodeSocketValueMenuFlag {

@@ -124,0 +125,4 @@
+
+const EnumPropertyItem *RNA_node_enum_definition_itemf(const RuntimeNodeEnumItems &enum_items,
+                                                       bool *r_free);
+const EnumPropertyItem *RNA_node_socket_enum_itemf(bContext *C,

@@ -0,0 +43,4 @@
+      break;
+    case SOCK_RGBA:
+      b.add_input<decl::Color>(name, identifier)
+          .default_value({0.8f, 0.8f, 0.8f, 1.0f})

@@ -0,0 +46,4 @@
+          .default_value({0.8f, 0.8f, 0.8f, 1.0f})
+          .supports_field();
+      break;
+    case SOCK_SHADER:

@@ -0,0 +53,4 @@
+      b.add_input<decl::Bool>(name, identifier).default_value(false).supports_field();
+      break;
+    case SOCK_INT:
+      b.add_input<decl::Int>(name, identifier).min(-100000).max(100000).supports_field();

@@ -0,0 +160,4 @@
+  /* Allow the node group interface to define the socket order. */
+  b.use_custom_socket_order();
+
+  const bool fields_type = ELEM(data_type,

@@ -0,0 +260,4 @@
+  else {
+    /* No sensible way to connect inputs currently:
+     * Switch socket connection will always create a conflicting enum ref.
+     * Case input sockets don't existing without an actual enum definition.

@@ -0,0 +265,4 @@
+  }
+}
+
+/* Multifunction which evaluates the switch input for each enum item and partially fills the output

@@ -0,0 +267,4 @@
+
+/* Multifunction which evaluates the switch input for each enum item and partially fills the output
+ * array with values from the input array where the identifier matches. */
+template<typename T> class MenuSwitchFn : public mf::MultiFunction {

@@ -0,0 +317,4 @@
+  bool can_be_field_ = false;
+  const NodeEnumDefinition &enum_def_;
+  const CPPType *cpp_type_;
+  std::unique_ptr<MultiFunction> multi_function_;

@@ -0,0 +380,4 @@
+    /* No guarantee that the switch input matches any enum,
+     * set default outputs to ensure valid state. */
+    if (!found_match) {
+      cpp_type_->value_initialize(output_ptr);

@@ -0,0 +387,4 @@
+
+  void execute_field(Field<int> condition, lf::Params &params) const
+  {
+    if (!condition.node().depends_on_input()) {

@@ -0,0 +500,4 @@
+                                               SOCK_GEOMETRY,
+                                               SOCK_OBJECT,
+                                               SOCK_COLLECTION,
+                                               SOCK_TEXTURE,

@@ -484,0 +513,4 @@
+
+bool Menu::can_connect(const bNodeSocket &socket) const
+{
+  return sockets_can_connect(*this, socket) && socket.type == SOCK_MENU;

@@ -804,0 +892,4 @@
+          if (input->is_available() && input->type == SOCK_MENU) {
+            for (const bNodeSocket *output : node->output_sockets()) {
+              this->update_socket_enum_definition(
+                  *input->default_value_typed<bNodeSocketValueMenu>(),

@@ -0,0 +256,4 @@
+    const NodeMenuSwitch &storage = node_storage(node);
+    const eNodeSocketDatatype data_type = eNodeSocketDatatype(storage.data_type);
+    can_be_field_ = ELEM(
+        data_type, SOCK_FLOAT, SOCK_INT, SOCK_BOOLEAN, SOCK_VECTOR, SOCK_RGBA, SOCK_ROTATION);

@@ -0,0 +313,4 @@
+    }
+    /* No guarantee that the switch input matches any enum,
+     * set default outputs to ensure valid state. */
+    set_default_remaining_node_outputs(params, node_);

@@ -1826,6 +1865,8 @@ static void socket_id_user_increment(bNodeSocket *sock)
     case SOCK_ROTATION:
     case SOCK_INT:
     case SOCK_STRING:
+      /* Note: Enum socket node tree is a weak reference and not user-counted. */

@@ -0,0 +54,4 @@
+  if (!this->items().contains_ptr(&item)) {
+    return false;
+  }
+  const int remove_index = &item - this->items().begin();

@@ -0,0 +75,4 @@
+    return;
+  }
+  this->items_num = 0;
+  MEM_SAFE_FREE(this->items_array);

@@ -78,6 +79,7 @@ template<> void socket_data_id_user_increment(bNodeSocketValueMaterial &data)
 {
   id_us_plus(reinterpret_cast<ID *>(data.value));
 }
+/* Note: bNodeSocketValueMenu does not do user counting on the node tree reference. */

@@ -484,6 +485,9 @@ class NodeTreeMainUpdater {
     this->make_node_previews_dirty(ntree);
 
     this->propagate_runtime_flags(ntree);
+    if (this->propagate_enum_definitions(ntree)) {

@@ -507,3 +507,3 @@
   const eNodeSocketDatatype socket_type = eNodeSocketDatatype(typeinfo->type);
 
-  /* The property should be created in #MOD_nodes_update_interface with the correct type. */
+  /* The property should be created in #MOD_nodes_update_properties with the correct type. */

@@ -995,2 +998,4 @@
           break;
         }
+        case SOCK_MENU:
+          uiItemL(sub, "MENU SOCKET TODO", ICON_NONE);

@@ -7941,7 +7941,6 @@ static void rna_def_modifier_grease_pencil_tint(BlenderRNA *brna)
   RNA_def_property_update(prop, 0, "rna_Modifier_update");
 }
 
-

@@ -68,7 +68,6 @@ void update_input_properties_from_node_tree(const bNodeTree &tree,
                                             const IDProperty *old_properties,
                                             bool use_bool_for_use_attribute,
                                             IDProperty &properties);
-

@@ -373,6 +373,7 @@ DefNode(GeometryNode, GEO_NODE_IS_VIEWPORT, 0, "IS_VIEWPORT", IsViewport, "Is Vi
 DefNode(GeometryNode, GEO_NODE_JOIN_GEOMETRY, 0, "JOIN_GEOMETRY", JoinGeometry, "Join Geometry", "Merge separately generated geometries into a single one")
 DefNode(GeometryNode, GEO_NODE_MATERIAL_SELECTION