Refactor: Use std::variant instead of union for asset data

Rather than a manually managed union, use `std::variant` which is
generally safer. E.g. an invalid access will now throw an exception
instead of causing undefined behavior (which may or may not crash, or
cause a data corruption). Code is also simplified this way.

Pull Request: https://projects.blender.org/blender/blender/pulls/125494

source/blender/asset_system/AS_asset_representation.hh

@@ -16,6 +16,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <variant>
 
 #include "BLI_string_ref.hh"
 #include "BLI_utility_mixins.hh"
@@ -38,21 +39,13 @@ class AssetRepresentation : NonCopyable, NonMovable {
    * within the asset library).
    */
   std::string relative_identifier_;
-  /**
-   * Indicate if this is a local or external asset, and as such, which of the union members below
-   * should be used.
-   */
-  const bool is_local_id_ = false;
 
   struct ExternalAsset {
     std::string name;
     int id_type = 0;
     std::unique_ptr<AssetMetaData> metadata_ = nullptr;
   };
-  union {
-    ExternalAsset external_asset_;
-    ID *local_asset_id_ = nullptr; /* Non-owning. */
-  };
+  std::variant<ExternalAsset, ID *> asset_;
 
   friend class AssetLibrary;
 
@@ -70,7 +63,7 @@ class AssetRepresentation : NonCopyable, NonMovable {
   AssetRepresentation(StringRef relative_asset_path,
                       ID &id,
                       const AssetLibrary &owner_asset_library);
-  ~AssetRepresentation();
+  ~AssetRepresentation() = default;
 
   /**
    * Create a weak reference for this asset that can be written to files, but can break under a

source/blender/asset_system/intern/asset_library.cc

@@ -232,7 +232,7 @@ void AssetLibrary::remap_ids_and_remove_invalid(const bke::id::IDRemapper &mappi
     AssetRepresentation &asset = *asset_ptr;
     BLI_assert(asset.is_local_id());
 
-    const IDRemapperApplyResult result = mappings.apply(&asset.local_asset_id_,
+    const IDRemapperApplyResult result = mappings.apply(&std::get<ID *>(asset.asset_),
                                                         ID_REMAP_APPLY_DEFAULT);
 
     /* Entirely remove assets whose ID is unset. We don't want assets with a null ID pointer. */

source/blender/asset_system/intern/asset_representation.cc

@@ -27,34 +27,20 @@ AssetRepresentation::AssetRepresentation(StringRef relative_path,
                                          const AssetLibrary &owner_asset_library)
     : owner_asset_library_(owner_asset_library),
       relative_identifier_(relative_path),
-      is_local_id_(false),
-      external_asset_()
+      asset_(AssetRepresentation::ExternalAsset{name, id_type, std::move(metadata)})
 {
-  external_asset_.name = name;
-  external_asset_.id_type = id_type;
-  external_asset_.metadata_ = std::move(metadata);
 }
 
 AssetRepresentation::AssetRepresentation(StringRef relative_path,
                                          ID &id,
                                          const AssetLibrary &owner_asset_library)
-    : owner_asset_library_(owner_asset_library),
-      relative_identifier_(relative_path),
-      is_local_id_(true),
-      local_asset_id_(&id)
+    : owner_asset_library_(owner_asset_library), relative_identifier_(relative_path), asset_(&id)
 {
   if (!id.asset_data) {
     throw std::invalid_argument("Passed ID is not an asset");
   }
 }
 
-AssetRepresentation::~AssetRepresentation()
-{
-  if (!is_local_id_) {
-    external_asset_.~ExternalAsset();
-  }
-}
-
 AssetWeakReference AssetRepresentation::make_weak_reference() const
 {
   return AssetWeakReference::make_reference(owner_asset_library_, relative_identifier_);
@@ -62,25 +48,26 @@ AssetWeakReference AssetRepresentation::make_weak_reference() const
 
 StringRefNull AssetRepresentation::get_name() const
 {
-  if (is_local_id_) {
-    return local_asset_id_->name + 2;
+  if (const ID *local_id = this->local_id()) {
+    return local_id->name + 2;
   }
-
-  return external_asset_.name;
+  return std::get<ExternalAsset>(asset_).name;
 }
 
 ID_Type AssetRepresentation::get_id_type() const
 {
-  if (is_local_id_) {
-    return GS(local_asset_id_->name);
+  if (const ID *local_id = this->local_id()) {
+    return GS(local_id->name);
   }
-
-  return ID_Type(external_asset_.id_type);
+  return ID_Type(std::get<ExternalAsset>(asset_).id_type);
 }
 
 AssetMetaData &AssetRepresentation::get_metadata() const
 {
-  return is_local_id_ ? *local_asset_id_->asset_data : *external_asset_.metadata_;
+  if (const ID *local_id = this->local_id()) {
+    return *local_id->asset_data;
+  }
+  return *std::get<ExternalAsset>(asset_).metadata_;
 }
 
 StringRefNull AssetRepresentation::library_relative_identifier() const
@@ -130,12 +117,12 @@ bool AssetRepresentation::get_use_relative_path() const
 
 ID *AssetRepresentation::local_id() const
 {
-  return is_local_id_ ? local_asset_id_ : nullptr;
+  return this->is_local_id() ? std::get<ID *>(asset_) : nullptr;
 }
 
 bool AssetRepresentation::is_local_id() const
 {
-  return is_local_id_;
+  return std::holds_alternative<ID *>(asset_);
 }
 
 const AssetLibrary &AssetRepresentation::owner_asset_library() const