Cleanup: Add & use enum value for ID Outliner element type

Code to check if the Outliner tree-element type was the general ID one
would always check against "0" (explicity or even implicitly). For
somebody unfamiliar with the code this is very confusing. Instead the
value should be given a name, e.g. through an enum.

Adds `TSE_SOME_ID` as the "default" ID tree-element type. Other types
may still represent IDs, as I explained in a comment at the definition.

There may also still be cases where the type is checked against "0". I
noted in the comment that such cases should be cleaned up if found.

17 files changed, 107 insertions, 85 deletions

diff --git a/source/blender/blenkernel/intern/outliner_treehash.c b/source/blender/blenkernel/intern/outliner_treehash.c
index 05873d20f7f..b9497d389e7 100644
--- a/source/blender/blenkernel/intern/outliner_treehash.c
+++ b/source/blender/blenkernel/intern/outliner_treehash.c
@@ -101,7 +101,7 @@ static unsigned int tse_hash(const void *ptr)
     unsigned int u_int;
   } hash;
 
-  BLI_assert(tse->type || !tse->nr);
+  BLI_assert((tse->type != TSE_SOME_ID) || !tse->nr);
 
   hash.h_pair[0] = tse->type;
   hash.h_pair[1] = tse->nr;
@@ -193,7 +193,7 @@ static TseGroup *BKE_outliner_treehash_lookup_group(GHash *th, short type, short
 {
   TreeStoreElem tse_template;
   tse_template.type = type;
-  tse_template.nr = type ? nr : 0; /* we're picky! :) */
+  tse_template.nr = (type == TSE_SOME_ID) ? 0 : nr; /* we're picky! :) */
   tse_template.id = id;
 
   BLI_assert(th);
diff --git a/source/blender/editors/space_outliner/outliner_collections.c b/source/blender/editors/space_outliner/outliner_collections.c
index ef5733fe375..d54e35f659c 100644
--- a/source/blender/editors/space_outliner/outliner_collections.c
+++ b/source/blender/editors/space_outliner/outliner_collections.c
@@ -71,7 +71,7 @@ bool outliner_is_collection_tree_element(const TreeElement *te)
            TSE_VIEW_COLLECTION_BASE)) {
     return true;
   }
-  if (tselem->type == 0 && te->idcode == ID_GR) {
+  if ((tselem->type == TSE_SOME_ID) && te->idcode == ID_GR) {
     return true;
   }
 
@@ -94,7 +94,7 @@ Collection *outliner_collection_from_tree_element(const TreeElement *te)
     Scene *scene = (Scene *)tselem->id;
     return scene->master_collection;
   }
-  if (tselem->type == 0 && te->idcode == ID_GR) {
+  if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_GR)) {
     return (Collection *)tselem->id;
   }
 
@@ -111,7 +111,7 @@ TreeTraversalAction outliner_find_selected_collections(TreeElement *te, void *cu
     return TRAVERSE_CONTINUE;
   }
 
-  if (tselem->type || (tselem->id && GS(tselem->id->name) != ID_GR)) {
+  if ((tselem->type != TSE_SOME_ID) || (tselem->id && GS(tselem->id->name) != ID_GR)) {
     return TRAVERSE_SKIP_CHILDS;
   }
 
@@ -127,7 +127,7 @@ TreeTraversalAction outliner_find_selected_objects(TreeElement *te, void *custom
     return TRAVERSE_CONTINUE;
   }
 
-  if (tselem->type || (tselem->id == NULL) || (GS(tselem->id->name) != ID_OB)) {
+  if ((tselem->type != TSE_SOME_ID) || (tselem->id == NULL) || (GS(tselem->id->name) != ID_OB)) {
     return TRAVERSE_SKIP_CHILDS;
   }
 
@@ -1458,7 +1458,7 @@ static TreeTraversalAction outliner_hide_find_data_to_edit(TreeElement *te, void
       BLI_gset_add(data->collections_to_edit, lc);
     }
   }
-  else if (tselem->type == 0 && te->idcode == ID_OB) {
+  else if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
     Object *ob = (Object *)tselem->id;
     Base *base = BKE_view_layer_base_find(data->view_layer, ob);
     BLI_gset_add(data->bases_to_edit, base);
diff --git a/source/blender/editors/space_outliner/outliner_context.c b/source/blender/editors/space_outliner/outliner_context.c
index e2b3b79e027..4293d8da73e 100644
--- a/source/blender/editors/space_outliner/outliner_context.c
+++ b/source/blender/editors/space_outliner/outliner_context.c
@@ -34,7 +34,7 @@ static void outliner_context_selected_ids_recursive(const ListBase *subtree,
 {
   LISTBASE_FOREACH (const TreeElement *, te, subtree) {
     const TreeStoreElem *tse = TREESTORE(te);
-    if ((tse->flag & TSE_SELECTED) && (ELEM(tse->type, 0, TSE_LAYER_COLLECTION))) {
+    if ((tse->flag & TSE_SELECTED) && (ELEM(tse->type, TSE_SOME_ID, TSE_LAYER_COLLECTION))) {
       CTX_data_id_list_add(result, tse->id);
     }
     outliner_context_selected_ids_recursive(&te->subtree, result);
diff --git a/source/blender/editors/space_outliner/outliner_dragdrop.c b/source/blender/editors/space_outliner/outliner_dragdrop.c
index 3090cab75ae..01fb0fc6f78 100644
--- a/source/blender/editors/space_outliner/outliner_dragdrop.c
+++ b/source/blender/editors/space_outliner/outliner_dragdrop.c
@@ -124,7 +124,7 @@ static ID *outliner_ID_drop_find(bContext *C, const wmEvent *event, short idcode
   TreeElement *te = outliner_drop_find(C, event);
   TreeStoreElem *tselem = (te) ? TREESTORE(te) : NULL;
 
-  if (te && te->idcode == idcode && tselem->type == 0) {
+  if (te && (te->idcode == idcode) && (tselem->type == TSE_SOME_ID)) {
     return tselem->id;
   }
   return NULL;
@@ -215,7 +215,7 @@ static bool is_collection_element(TreeElement *te)
 static bool is_object_element(TreeElement *te)
 {
   TreeStoreElem *tselem = TREESTORE(te);
-  return tselem->type == 0 && te->idcode == ID_OB;
+  return (tselem->type == TSE_SOME_ID) && te->idcode == ID_OB;
 }
 
 static bool is_pchan_element(TreeElement *te)
@@ -281,7 +281,7 @@ static int outliner_get_insert_index(TreeElement *drag_te,
 static bool parent_drop_allowed(TreeElement *te, Object *potential_child)
 {
   TreeStoreElem *tselem = TREESTORE(te);
-  if (te->idcode != ID_OB || tselem->type != 0) {
+  if ((te->idcode != ID_OB) || (tselem->type != TSE_SOME_ID)) {
     return false;
   }
 
@@ -421,7 +421,7 @@ static int parent_drop_invoke(bContext *C, wmOperator *op, const wmEvent *event)
   TreeElement *te = outliner_drop_find(C, event);
   TreeStoreElem *tselem = te ? TREESTORE(te) : NULL;
 
-  if (!(te && te->idcode == ID_OB && tselem->type == 0)) {
+  if (!(te && (te->idcode == ID_OB) && (tselem->type == TSE_SOME_ID))) {
     return OPERATOR_CANCELLED;
   }
 
diff --git a/source/blender/editors/space_outliner/outliner_draw.c b/source/blender/editors/space_outliner/outliner_draw.c
index 008ae727947..690adb09570 100644
--- a/source/blender/editors/space_outliner/outliner_draw.c
+++ b/source/blender/editors/space_outliner/outliner_draw.c
@@ -675,7 +675,7 @@ static void namebutton_fn(bContext *C, void *tsep, char *oldname)
   if (ts && tselem) {
     TreeElement *te = outliner_find_tree_element(&space_outliner->tree, tselem);
 
-    if (tselem->type == 0) {
+    if (tselem->type == TSE_SOME_ID) {
       BLI_libblock_ensure_unique_name(bmain, tselem->id->name);
 
       switch (GS(tselem->id->name)) {
@@ -1100,11 +1100,11 @@ static void outliner_draw_restrictbuts(uiBlock *block,
           UI_but_drawflag_enable(bt, UI_BUT_ICON_REVERSE);
         }
       }
-      else if ((tselem->type == 0 && te->idcode == ID_OB) &&
+      else if (((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) &&
                (te->flag & TE_CHILD_NOT_IN_COLLECTION)) {
         /* Don't show restrict columns for children that are not directly inside the collection. */
       }
-      else if (tselem->type == 0 && te->idcode == ID_OB) {
+      else if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
         PointerRNA ptr;
         Object *ob = (Object *)tselem->id;
         RNA_id_pointer_create(&ob->id, &ptr);
@@ -1699,7 +1699,7 @@ static void outliner_draw_userbuts(uiBlock *block,
   LISTBASE_FOREACH (TreeElement *, te, lb) {
     TreeStoreElem *tselem = TREESTORE(te);
     if (te->ys + 2 * UI_UNIT_Y >= region->v2d.cur.ymin && te->ys <= region->v2d.cur.ymax) {
-      if (tselem->type == 0) {
+      if (tselem->type == TSE_SOME_ID) {
         uiBut *bt;
         ID *id = tselem->id;
         const char *tip = NULL;
@@ -1949,7 +1949,7 @@ static void outliner_draw_mode_column_toggle(uiBlock *block,
                                              TreeStoreElem *tselem,
                                              const bool lock_object_modes)
 {
-  if (tselem->type != 0 || te->idcode != ID_OB) {
+  if ((tselem->type != TSE_SOME_ID) || (te->idcode != ID_OB)) {
     return;
   }
 
@@ -2046,7 +2046,7 @@ TreeElementIcon tree_element_get_icon(TreeStoreElem *tselem, TreeElement *te)
 {
   TreeElementIcon data = {0};
 
-  if (tselem->type) {
+  if (tselem->type != TSE_SOME_ID) {
     switch (tselem->type) {
       case TSE_ANIM_DATA:
         data.icon = ICON_ANIM_DATA; /* XXX */
@@ -2825,7 +2825,8 @@ int tree_element_id_type_to_index(TreeElement *te)
 {
   TreeStoreElem *tselem = TREESTORE(te);
 
-  const int id_index = tselem->type == 0 ? BKE_idtype_idcode_to_index(te->idcode) : INDEX_ID_GR;
+  const int id_index = (tselem->type == TSE_SOME_ID) ? BKE_idtype_idcode_to_index(te->idcode) :
+                                                       INDEX_ID_GR;
   if (id_index < INDEX_ID_OB) {
     return id_index;
   }
@@ -2862,9 +2863,9 @@ static void outliner_draw_iconrow(bContext *C,
     te->flag &= ~(TE_ICONROW | TE_ICONROW_MERGED);
 
     /* object hierarchy always, further constrained on level */
-    if (level < 1 || (tselem->type == 0 && te->idcode == ID_OB)) {
+    if ((level < 1) || ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB))) {
       /* active blocks get white circle */
-      if (tselem->type == 0) {
+      if (tselem->type == TSE_SOME_ID) {
         if (te->idcode == ID_OB) {
           active = (tvc->obact == (Object *)tselem->id) ? OL_DRAWSEL_NORMAL : OL_DRAWSEL_NONE;
         }
@@ -2879,7 +2880,7 @@ static void outliner_draw_iconrow(bContext *C,
         active = tree_element_type_active_state_get(C, tvc, te, tselem);
       }
 
-      if (!ELEM(tselem->type, 0, TSE_LAYER_COLLECTION, TSE_R_LAYER, TSE_GP_LAYER)) {
+      if (!ELEM(tselem->type, TSE_SOME_ID, TSE_LAYER_COLLECTION, TSE_R_LAYER, TSE_GP_LAYER)) {
         outliner_draw_iconrow_doit(block, te, fstyle, xmax, offsx, ys, alpha_fac, active, 1);
       }
       else {
@@ -2954,7 +2955,7 @@ static bool element_should_draw_faded(const TreeViewContext *tvc,
                                       const TreeElement *te,
                                       const TreeStoreElem *tselem)
 {
-  if (tselem->type == 0) {
+  if (tselem->type == TSE_SOME_ID) {
     switch (te->idcode) {
       case ID_OB: {
         const Object *ob = (const Object *)tselem->id;
@@ -3023,7 +3024,7 @@ static void outliner_draw_tree_element(bContext *C,
     GPU_blend(GPU_BLEND_ALPHA);
 
     /* Colors for active/selected data. */
-    if (tselem->type == 0) {
+    if (tselem->type == TSE_SOME_ID) {
       if (te->idcode == ID_OB) {
         Object *ob = (Object *)tselem->id;
         Base *base = (te->directdata) ? (Base *)te->directdata :
@@ -3080,7 +3081,7 @@ static void outliner_draw_tree_element(bContext *C,
     if (tselem->type == TSE_VIEW_COLLECTION_BASE) {
       /* Scene collection in view layer can't expand/collapse. */
     }
-    else if (te->subtree.first || (tselem->type == 0 && te->idcode == ID_SCE) ||
+    else if (te->subtree.first || ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_SCE)) ||
              (te->flag & TE_LAZY_CLOSED)) {
       /* Open/close icon, only when sub-levels, except for scene. */
       int icon_x = startx;
@@ -3117,7 +3118,7 @@ static void outliner_draw_tree_element(bContext *C,
       offsx += 2 * ufac;
     }
 
-    if (ELEM(tselem->type, 0, TSE_LAYER_COLLECTION) ||
+    if (ELEM(tselem->type, TSE_SOME_ID, TSE_LAYER_COLLECTION) ||
         ((tselem->type == TSE_RNA_STRUCT) && RNA_struct_is_ID(te->rnaptr.type))) {
       const BIFIconID lib_icon = UI_icon_from_library(tselem->id);
       if (lib_icon != ICON_NONE) {
@@ -3143,7 +3144,7 @@ static void outliner_draw_tree_element(bContext *C,
     /* Closed item, we draw the icons, not when it's a scene, or master-server list though. */
     if (!TSELEM_OPEN(tselem, space_outliner)) {
       if (te->subtree.first) {
-        if (tselem->type == 0 && te->idcode == ID_SCE) {
+        if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_SCE)) {
           /* Pass. */
         }
         /* this tree element always has same amount of branches, so don't draw */
@@ -3210,7 +3211,7 @@ static bool subtree_contains_object(ListBase *lb)
 {
   LISTBASE_FOREACH (TreeElement *, te, lb) {
     TreeStoreElem *tselem = TREESTORE(te);
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
       return true;
     }
   }
@@ -3265,7 +3266,7 @@ static void outliner_draw_hierarchy_lines_recursive(uint pos,
 
         y = *starty;
       }
-      else if (tselem->type == 0 && te->idcode == ID_OB) {
+      else if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
         if (subtree_contains_object(&te->subtree)) {
           draw_hierarchy_line = true;
           is_object_line = true;
diff --git a/source/blender/editors/space_outliner/outliner_edit.c b/source/blender/editors/space_outliner/outliner_edit.c
index 09aa268d856..c63209b6b60 100644
--- a/source/blender/editors/space_outliner/outliner_edit.c
+++ b/source/blender/editors/space_outliner/outliner_edit.c
@@ -638,7 +638,7 @@ static bool outliner_id_remap_find_tree_element(bContext *C,
     if (y > te->ys && y < te->ys + UI_UNIT_Y) {
       TreeStoreElem *tselem = TREESTORE(te);
 
-      if (tselem->type == 0 && tselem->id) {
+      if ((tselem->type == TSE_SOME_ID) && tselem->id) {
         printf("found id %s (%p)!\n", tselem->id->name, tselem->id);
 
         RNA_enum_set(op->ptr, "id_type", GS(tselem->id->name));
@@ -763,7 +763,7 @@ static int outliner_id_copy_tag(SpaceOutliner *space_outliner, ListBase *tree)
     TreeStoreElem *tselem = TREESTORE(te);
 
     /* if item is selected and is an ID, tag it as needing to be copied. */
-    if (tselem->flag & TSE_SELECTED && ELEM(tselem->type, 0, TSE_LAYER_COLLECTION)) {
+    if (tselem->flag & TSE_SELECTED && ELEM(tselem->type, TSE_SOME_ID, TSE_LAYER_COLLECTION)) {
       ID *id = tselem->id;
       if (!(id->tag & LIB_TAG_DOIT)) {
         BKE_copybuffer_tag_ID(tselem->id);
@@ -1640,7 +1640,7 @@ static int subtree_has_objects(ListBase *lb)
 {
   LISTBASE_FOREACH (TreeElement *, te, lb) {
     TreeStoreElem *tselem = TREESTORE(te);
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
       return 1;
     }
     if (subtree_has_objects(&te->subtree)) {
@@ -1658,7 +1658,7 @@ static void tree_element_show_hierarchy(Scene *scene, SpaceOutliner *space_outli
     TreeStoreElem *tselem = TREESTORE(te);
 
     if (ELEM(tselem->type,
-             0,
+             TSE_SOME_ID,
              TSE_SCENE_OBJECTS_BASE,
              TSE_VIEW_COLLECTION_BASE,
              TSE_LAYER_COLLECTION)) {
diff --git a/source/blender/editors/space_outliner/outliner_select.c b/source/blender/editors/space_outliner/outliner_select.c
index e31af48ab7e..d53a37fa60e 100644
--- a/source/blender/editors/space_outliner/outliner_select.c
+++ b/source/blender/editors/space_outliner/outliner_select.c
@@ -198,7 +198,7 @@ void outliner_item_mode_toggle(bContext *C,
 {
   TreeStoreElem *tselem = TREESTORE(te);
 
-  if (tselem->type == 0 && te->idcode == ID_OB) {
+  if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
     Object *ob = (Object *)tselem->id;
     Base *base = BKE_view_layer_base_find(tvc->view_layer, ob);
 
@@ -301,7 +301,7 @@ static void tree_element_object_activate(bContext *C,
   Object *ob = NULL;
 
   /* if id is not object, we search back */
-  if (tselem->type == 0 && te->idcode == ID_OB) {
+  if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
     ob = (Object *)tselem->id;
   }
   else {
@@ -443,7 +443,7 @@ static void tree_element_world_activate(bContext *C, Scene *scene, TreeElement *
   TreeElement *tep = te->parent;
   if (tep) {
     TreeStoreElem *tselem = TREESTORE(tep);
-    if (tselem->type == 0) {
+    if (tselem->type == TSE_SOME_ID) {
       sce = (Scene *)tselem->id;
     }
   }
@@ -1165,7 +1165,7 @@ static void outliner_set_properties_tab(bContext *C, TreeElement *te, TreeStoreE
   int context = 0;
 
   /* ID Types */
-  if (tselem->type == 0) {
+  if (tselem->type == TSE_SOME_ID) {
     RNA_id_pointer_create(tselem->id, &ptr);
 
     switch (te->idcode) {
@@ -1374,12 +1374,12 @@ static void do_outliner_item_activate_tree_element(bContext *C,
                                  tvc->scene,
                                  tvc->view_layer,
                                  te,
-                                 (extend && tselem->type == 0) ? OL_SETSEL_EXTEND :
-                                                                 OL_SETSEL_NORMAL,
-                                 recursive && tselem->type == 0);
+                                 (extend && tselem->type == TSE_SOME_ID) ? OL_SETSEL_EXTEND :
+                                                                           OL_SETSEL_NORMAL,
+                                 recursive && tselem->type == TSE_SOME_ID);
   }
 
-  if (tselem->type == 0) { /* The lib blocks. */
+  if (tselem->type == TSE_SOME_ID) { /* The lib blocks. */
     if (do_activate_data == false) {
       /* Only select in outliner. */
     }
diff --git a/source/blender/editors/space_outliner/outliner_sync.c b/source/blender/editors/space_outliner/outliner_sync.c
index 8bd5e3a130a..6543a909a41 100644
--- a/source/blender/editors/space_outliner/outliner_sync.c
+++ b/source/blender/editors/space_outliner/outliner_sync.c
@@ -326,7 +326,7 @@ static void outliner_sync_selection_from_outliner(Scene *scene,
   LISTBASE_FOREACH (TreeElement *, te, tree) {
     TreeStoreElem *tselem = TREESTORE(te);
 
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
       if (sync_types->object) {
         outliner_select_sync_to_object(view_layer, te, tselem, selected_items->objects);
       }
@@ -503,7 +503,7 @@ static void outliner_sync_selection_to_outliner(ViewLayer *view_layer,
   LISTBASE_FOREACH (TreeElement *, te, tree) {
     TreeStoreElem *tselem = TREESTORE(te);
 
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && te->idcode == ID_OB) {
       if (sync_types->object) {
         outliner_select_sync_from_object(view_layer, active_data->object, te, tselem);
       }
diff --git a/source/blender/editors/space_outliner/outliner_tools.c b/source/blender/editors/space_outliner/outliner_tools.c
index b735064cfef..9af2ba6a82b 100644
--- a/source/blender/editors/space_outliner/outliner_tools.c
+++ b/source/blender/editors/space_outliner/outliner_tools.c
@@ -106,7 +106,7 @@ static void get_element_operation_type(
   TreeStoreElem *tselem = TREESTORE(te);
   if (tselem->flag & TSE_SELECTED) {
     /* Layer collection points to collection ID. */
-    if (!ELEM(tselem->type, 0, TSE_LAYER_COLLECTION)) {
+    if (!ELEM(tselem->type, TSE_SOME_ID, TSE_LAYER_COLLECTION)) {
       if (*datalevel == 0) {
         *datalevel = tselem->type;
       }
@@ -402,7 +402,8 @@ static void outliner_do_libdata_operation(bContext *C,
   LISTBASE_FOREACH (TreeElement *, te, lb) {
     TreeStoreElem *tselem = TREESTORE(te);
     if (tselem->flag & TSE_SELECTED) {
-      if ((tselem->type == 0 && te->idcode != 0) || tselem->type == TSE_LAYER_COLLECTION) {
+      if (((tselem->type == TSE_SOME_ID) && (te->idcode != 0)) ||
+          tselem->type == TSE_LAYER_COLLECTION) {
         TreeStoreElem *tsep = te->parent ? TREESTORE(te->parent) : NULL;
         operation_fn(C, reports, scene, te, tsep, tselem, user_data);
       }
@@ -1044,7 +1045,7 @@ void outliner_do_object_operation_ex(bContext *C,
     TreeStoreElem *tselem = TREESTORE(te);
     bool select_handled = false;
     if (tselem->flag & TSE_SELECTED) {
-      if (tselem->type == 0 && te->idcode == ID_OB) {
+      if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
         /* When objects selected in other scenes... dunno if that should be allowed. */
         Scene *scene_owner = (Scene *)outliner_search_back(te, ID_SCE);
         if (scene_owner && scene_act != scene_owner) {
@@ -1601,7 +1602,7 @@ static TreeTraversalAction outliner_find_objects_to_delete(TreeElement *te, void
     return TRAVERSE_CONTINUE;
   }
 
-  if (tselem->type || (tselem->id == NULL) || (GS(tselem->id->name) != ID_OB)) {
+  if ((tselem->type != TSE_SOME_ID) || (tselem->id == NULL) || (GS(tselem->id->name) != ID_OB)) {
     return TRAVERSE_SKIP_CHILDS;
   }
 
diff --git a/source/blender/editors/space_outliner/outliner_tree.c b/source/blender/editors/space_outliner/outliner_tree.c
index 0d82ba992c2..3696ef8dbce 100644
--- a/source/blender/editors/space_outliner/outliner_tree.c
+++ b/source/blender/editors/space_outliner/outliner_tree.c
@@ -307,7 +307,7 @@ static void outliner_add_line_styles(SpaceOutliner *space_outliner,
           continue;
         }
         linestyle->id.tag &= ~LIB_TAG_DOIT;
-        outliner_add_element(space_outliner, lb, linestyle, te, 0, 0);
+        outliner_add_element(space_outliner, lb, linestyle, te, TSE_SOME_ID, 0);
       }
     }
   }
@@ -332,7 +332,7 @@ static void outliner_add_scene_contents(SpaceOutliner *space_outliner,
   }
 
   /* World */
-  outliner_add_element(space_outliner, lb, sce->world, te, 0, 0);
+  outliner_add_element(space_outliner, lb, sce->world, te, TSE_SOME_ID, 0);
 
   /* Collections */
   ten = outliner_add_element(space_outliner, lb, &sce->id, te, TSE_SCENE_COLLECTION_BASE, 0);
@@ -343,7 +343,7 @@ static void outliner_add_scene_contents(SpaceOutliner *space_outliner,
   ten = outliner_add_element(space_outliner, lb, sce, te, TSE_SCENE_OBJECTS_BASE, 0);
   ten->name = IFACE_("Objects");
   FOREACH_SCENE_OBJECT_BEGIN (sce, ob) {
-    outliner_add_element(space_outliner, &ten->subtree, ob, ten, 0, 0);
+    outliner_add_element(space_outliner, &ten->subtree, ob, ten, TSE_SOME_ID, 0);
   }
   FOREACH_SCENE_OBJECT_END;
   outliner_make_object_parent_hierarchy(&ten->subtree);
@@ -368,14 +368,14 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
                        &te->subtree,
                        ob->poselib,
                        te,
-                       0,
+                       TSE_SOME_ID,
                        0); /* XXX FIXME.. add a special type for this. */
 
   if (ob->proxy && !ID_IS_LINKED(ob)) {
     outliner_add_element(space_outliner, &te->subtree, ob->proxy, te, TSE_PROXY, 0);
   }
 
-  outliner_add_element(space_outliner, &te->subtree, ob->data, te, 0, 0);
+  outliner_add_element(space_outliner, &te->subtree, ob->data, te, TSE_SOME_ID, 0);
 
   if (ob->pose) {
     bArmature *arm = ob->data;
@@ -458,7 +458,7 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
   }
 
   for (int a = 0; a < ob->totcol; a++) {
-    outliner_add_element(space_outliner, &te->subtree, ob->mat[a], te, 0, a);
+    outliner_add_element(space_outliner, &te->subtree, ob->mat[a], te, TSE_SOME_ID, a);
   }
 
   if (!BLI_listbase_is_empty(&ob->constraints)) {
@@ -624,7 +624,8 @@ static void outliner_add_object_contents(SpaceOutliner *space_outliner,
 
   /* duplicated group */
   if (ob->instance_collection && (ob->transflag & OB_DUPLICOLLECTION)) {
-    outliner_add_element(space_outliner, &te->subtree, ob->instance_collection, te, 0, 0);
+    outliner_add_element(
+        space_outliner, &te->subtree, ob->instance_collection, te, TSE_SOME_ID, 0);
   }
 }
 
@@ -686,9 +687,9 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
         outliner_add_element(space_outliner, &te->subtree, me, te, TSE_ANIM_DATA, 0);
       }
 
-      outliner_add_element(space_outliner, &te->subtree, me->key, te, 0, 0);
+      outliner_add_element(space_outliner, &te->subtree, me->key, te, TSE_SOME_ID, 0);
       for (int a = 0; a < me->totcol; a++) {
-        outliner_add_element(space_outliner, &te->subtree, me->mat[a], te, 0, a);
+        outliner_add_element(space_outliner, &te->subtree, me->mat[a], te, TSE_SOME_ID, a);
       }
       /* could do tfaces with image links, but the images are not grouped nicely.
        * would require going over all tfaces, sort images in use. etc... */
@@ -702,7 +703,7 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       }
 
       for (int a = 0; a < cu->totcol; a++) {
-        outliner_add_element(space_outliner, &te->subtree, cu->mat[a], te, 0, a);
+        outliner_add_element(space_outliner, &te->subtree, cu->mat[a], te, TSE_SOME_ID, a);
       }
       break;
     }
@@ -714,7 +715,7 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       }
 
       for (int a = 0; a < mb->totcol; a++) {
-        outliner_add_element(space_outliner, &te->subtree, mb->mat[a], te, 0, a);
+        outliner_add_element(space_outliner, &te->subtree, mb->mat[a], te, TSE_SOME_ID, a);
       }
       break;
     }
@@ -730,7 +731,7 @@ static void outliner_add_id_contents(SpaceOutliner *space_outliner,
       if (outliner_animdata_test(tex->adt)) {
         outliner_add_element(space_outliner, &te->subtree, tex, te, TSE_ANIM_DATA, 0);
       }
-      outliner_add_element(space_outliner, &te->subtree, tex->ima, te, 0, 0);
+      outliner_add_element(space_outliner, &te->subtree, tex->ima, te, TSE_SOME_ID, 0);
       break;
     }
     case ID_CA: {
@@ -1008,7 +1009,7 @@ TreeElement *outliner_add_element(SpaceOutliner *space_outliner,
   if (te->type) {
     outliner_tree_element_type_expand(te->type, space_outliner);
   }
-  else if (type == 0) {
+  else if (type == TSE_SOME_ID) {
     TreeStoreElem *tsepar = parent ? TREESTORE(parent) : NULL;
 
     /* ID data-block. */
@@ -1229,7 +1230,7 @@ BLI_INLINE void outliner_add_collection_objects(SpaceOutliner *space_outliner,
                                                 TreeElement *parent)
 {
   LISTBASE_FOREACH (CollectionObject *, cob, &collection->gobject) {
-    outliner_add_element(space_outliner, tree, cob->ob, parent, 0, 0);
+    outliner_add_element(space_outliner, tree, cob->ob, parent, TSE_SOME_ID, 0);
   }
 }
 
@@ -1240,7 +1241,8 @@ static TreeElement *outliner_add_collection_recursive(SpaceOutliner *space_outli
   outliner_add_collection_init(ten, collection);
 
   LISTBASE_FOREACH (CollectionChild *, child, &collection->children) {
-    outliner_add_element(space_outliner, &ten->subtree, &child->collection->id, ten, 0, 0);
+    outliner_add_element(
+        space_outliner, &ten->subtree, &child->collection->id, ten, TSE_SOME_ID, 0);
   }
 
   if (space_outliner->outlinevis != SO_SCENES) {
@@ -1265,7 +1267,7 @@ void outliner_make_object_parent_hierarchy(ListBase *lb)
     TreeElement *ten = te->next;
     TreeStoreElem *tselem = TREESTORE(te);
 
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && te->idcode == ID_OB) {
       Object *ob = (Object *)tselem->id;
       if (ob->parent && ob->parent->id.newid) {
         BLI_remlink(lb, te);
@@ -1406,7 +1408,7 @@ static void outliner_sort(ListBase *lb)
 
   /* sorting rules; only object lists, ID lists, or deformgroups */
   if (ELEM(tselem->type, TSE_DEFGROUP, TSE_ID_BASE) ||
-      (tselem->type == 0 && te->idcode == ID_OB)) {
+      ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB))) {
     int totelem = BLI_listbase_count(lb);
 
     if (totelem > 1) {
@@ -1420,7 +1422,7 @@ static void outliner_sort(ListBase *lb)
         tp->name = te->name;
         tp->idcode = te->idcode;
 
-        if (tselem->type && tselem->type != TSE_DEFGROUP) {
+        if ((tselem->type != TSE_SOME_ID) && tselem->type != TSE_DEFGROUP) {
           tp->idcode = 0; /* Don't sort this. */
         }
         if (tselem->type == TSE_ID_BASE) {
@@ -1471,7 +1473,7 @@ static void outliner_collections_children_sort(ListBase *lb)
   TreeStoreElem *tselem = TREESTORE(te);
 
   /* Sorting rules: only object lists. */
-  if (tselem->type == 0 && te->idcode == ID_OB) {
+  if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
     int totelem = BLI_listbase_count(lb);
 
     if (totelem > 1) {
@@ -1546,7 +1548,7 @@ static bool test_collection_callback(TreeElement *te)
 static bool test_object_callback(TreeElement *te)
 {
   TreeStoreElem *tselem = TREESTORE(te);
-  return ((tselem->type == 0) && (te->idcode == ID_OB));
+  return ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB));
 }
 
 /**
@@ -1707,7 +1709,7 @@ static bool outliner_element_visible_get(ViewLayer *view_layer,
   }
 
   TreeStoreElem *tselem = TREESTORE(te);
-  if ((tselem->type == 0) && (te->idcode == ID_OB)) {
+  if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
     if ((exclude_filter & SO_FILTER_OB_TYPE) == SO_FILTER_OB_TYPE) {
       return false;
     }
@@ -1790,14 +1792,15 @@ static bool outliner_element_visible_get(ViewLayer *view_layer,
       return is_visible;
     }
 
-    if ((te->parent != NULL) && (TREESTORE(te->parent)->type == 0) &&
+    if ((te->parent != NULL) && (TREESTORE(te->parent)->type == TSE_SOME_ID) &&
         (te->parent->idcode == ID_OB)) {
       if (exclude_filter & SO_FILTER_NO_CHILDREN) {
         return false;
       }
     }
   }
-  else if (te->parent != NULL && TREESTORE(te->parent)->type == 0 && te->parent->idcode == ID_OB) {
+  else if ((te->parent != NULL) && (TREESTORE(te->parent)->type == TSE_SOME_ID) &&
+           (te->parent->idcode == ID_OB)) {
     if (exclude_filter & SO_FILTER_NO_OB_CONTENT) {
       return false;
     }
@@ -1821,7 +1824,7 @@ static bool outliner_element_is_collection_or_object(TreeElement *te)
 {
   TreeStoreElem *tselem = TREESTORE(te);
 
-  if ((tselem->type == 0) && (te->idcode == ID_OB)) {
+  if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
     return true;
   }
 
diff --git a/source/blender/editors/space_outliner/outliner_utils.c b/source/blender/editors/space_outliner/outliner_utils.c
index 92178cfdfc9..562457c62e9 100644
--- a/source/blender/editors/space_outliner/outliner_utils.c
+++ b/source/blender/editors/space_outliner/outliner_utils.c
@@ -226,7 +226,7 @@ TreeElement *outliner_find_id(SpaceOutliner *space_outliner, ListBase *lb, const
 {
   LISTBASE_FOREACH (TreeElement *, te, lb) {
     TreeStoreElem *tselem = TREESTORE(te);
-    if (tselem->type == 0) {
+    if (tselem->type == TSE_SOME_ID) {
       if (tselem->id == id) {
         return te;
       }
@@ -266,7 +266,7 @@ TreeElement *outliner_find_editbone(ListBase *lb, const EditBone *ebone)
     }
 
     TreeStoreElem *tselem = TREESTORE(te);
-    if (ELEM(tselem->type, 0, TSE_EBONE)) {
+    if (ELEM(tselem->type, TSE_SOME_ID, TSE_EBONE)) {
       TreeElement *tes = outliner_find_editbone(&te->subtree, ebone);
       if (tes) {
         return tes;
@@ -283,7 +283,7 @@ TreeElement *outliner_search_back_te(TreeElement *te, short idcode)
 
   while (te) {
     tselem = TREESTORE(te);
-    if (tselem->type == 0 && te->idcode == idcode) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == idcode)) {
       return te;
     }
     te = te->parent;
@@ -510,7 +510,7 @@ Base *ED_outliner_give_base_under_cursor(bContext *C, const int mval[2])
   te = outliner_find_item_at_y(space_outliner, &space_outliner->tree, view_mval[1]);
   if (te) {
     TreeStoreElem *tselem = TREESTORE(te);
-    if ((tselem->type == 0) && (te->idcode == ID_OB)) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
       Object *ob = (Object *)tselem->id;
       base = (te->directdata) ? (Base *)te->directdata : BKE_view_layer_base_find(view_layer, ob);
     }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_libraries.cc b/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
index a81ce11498a..91b690d35fa 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_libraries.cc
@@ -144,7 +144,7 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar,
       if (!tenlib) {
         /* Create library tree element on demand, depending if there are any data-blocks. */
         if (lib) {
-          tenlib = outliner_add_element(&space_outliner_, &lb, lib, nullptr, 0, 0);
+          tenlib = outliner_add_element(&space_outliner_, &lb, lib, nullptr, TSE_SOME_ID, 0);
         }
         else {
           tenlib = outliner_add_element(&space_outliner_, &lb, &mainvar, nullptr, TSE_ID_BASE, 0);
@@ -168,7 +168,7 @@ TreeElement *TreeDisplayLibraries::add_library_contents(Main &mainvar,
 
         for (ID *id : List<ID>(lbarray[a])) {
           if (library_id_filter_poll(lib, id)) {
-            outliner_add_element(&space_outliner_, &ten->subtree, id, ten, 0, 0);
+            outliner_add_element(&space_outliner_, &ten->subtree, id, ten, TSE_SOME_ID, 0);
           }
         }
       }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc b/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
index 559cb289f3f..69ccf014642 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_orphaned.cc
@@ -76,7 +76,8 @@ ListBase TreeDisplayIDOrphans::buildTree(const TreeSourceData &source_data)
     /* Add the orphaned data-blocks - these will not be added with any subtrees attached. */
     for (ID *id : List<ID>(lbarray[a])) {
       if (ID_REAL_USERS(id) <= 0) {
-        outliner_add_element(&space_outliner_, (te) ? &te->subtree : &tree, id, te, 0, 0);
+        outliner_add_element(
+            &space_outliner_, (te) ? &te->subtree : &tree, id, te, TSE_SOME_ID, 0);
       }
     }
   }
diff --git a/source/blender/editors/space_outliner/tree/tree_display_scenes.cc b/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
index f377512d81e..390f81cfcd1 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_scenes.cc
@@ -46,7 +46,8 @@ ListBase TreeDisplayScenes::buildTree(const TreeSourceData &source_data)
 
   for (ID *id : List<ID>(source_data.bmain->scenes)) {
     Scene *scene = reinterpret_cast<Scene *>(id);
-    TreeElement *te = outliner_add_element(&space_outliner_, &tree, scene, nullptr, 0, 0);
+    TreeElement *te = outliner_add_element(
+        &space_outliner_, &tree, scene, nullptr, TSE_SOME_ID, 0);
     TreeStoreElem *tselem = TREESTORE(te);
 
     /* New scene elements open by default */
diff --git a/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc b/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
index a0ebac5f451..89c9960a24f 100644
--- a/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
+++ b/source/blender/editors/space_outliner/tree/tree_display_view_layer.cc
@@ -80,7 +80,7 @@ ListBase TreeDisplayViewLayer::buildTree(const TreeSourceData &source_data)
     /* Show objects in the view layer. */
     for (Base *base : List<Base>(view_layer_->object_bases)) {
       TreeElement *te_object = outliner_add_element(
-          &space_outliner_, &tree, base->object, nullptr, 0, 0);
+          &space_outliner_, &tree, base->object, nullptr, TSE_SOME_ID, 0);
       te_object->directdata = base;
     }
 
@@ -158,7 +158,7 @@ void TreeDisplayViewLayer::add_layer_collection_objects(ListBase &tree,
   for (CollectionObject *cob : List<CollectionObject>(lc.collection->gobject)) {
     Base *base = BKE_view_layer_base_find(view_layer_, cob->ob);
     TreeElement *te_object = outliner_add_element(
-        &space_outliner_, &tree, base->object, &ten, 0, 0);
+        &space_outliner_, &tree, base->object, &ten, TSE_SOME_ID, 0);
     te_object->directdata = base;
   }
 }
@@ -203,7 +203,7 @@ void ObjectsChildrenBuilder::object_tree_elements_lookup_create_recursive(TreeEl
       continue;
     }
 
-    if (tselem->type == 0 && te->idcode == ID_OB) {
+    if ((tselem->type == TSE_SOME_ID) && (te->idcode == ID_OB)) {
       Object *ob = (Object *)tselem->id;
       /* Lookup children or add new, empty children vector. */
       Vector<TreeElement *> &tree_elements = object_tree_elements_map_.lookup_or_add(ob, {});
@@ -261,8 +261,12 @@ void ObjectsChildrenBuilder::make_object_parent_hierarchy_collections()
       if (!found) {
         /* We add the child in the tree even if it is not in the collection.
          * We deliberately clear its sub-tree though, to make it less prominent. */
-        TreeElement *child_ob_tree_element = outliner_add_element(
-            &outliner_, &parent_ob_tree_element->subtree, child, parent_ob_tree_element, 0, 0);
+        TreeElement *child_ob_tree_element = outliner_add_element(&outliner_,
+                                                                  &parent_ob_tree_element->subtree,
+                                                                  child,
+                                                                  parent_ob_tree_element,
+                                                                  TSE_SOME_ID,
+                                                                  0);
         outliner_free_tree(&child_ob_tree_element->subtree);
         child_ob_tree_element->flag |= TE_CHILD_NOT_IN_COLLECTION;
         child_ob_tree_elements.append(child_ob_tree_element);
diff --git a/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc b/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
index 13a25800800..5a9568ea906 100644
--- a/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
+++ b/source/blender/editors/space_outliner/tree/tree_element_anim_data.cc
@@ -44,7 +44,8 @@ TreeElementAnimData::TreeElementAnimData(TreeElement &legacy_te, ID &id)
 void TreeElementAnimData::expand(SpaceOutliner &space_outliner) const
 {
   /* Animation data-block itself. */
-  outliner_add_element(&space_outliner, &legacy_te_.subtree, anim_data_.action, &legacy_te_, 0, 0);
+  outliner_add_element(
+      &space_outliner, &legacy_te_.subtree, anim_data_.action, &legacy_te_, TSE_SOME_ID, 0);
 
   expand_drivers(space_outliner);
   expand_NLA_tracks(space_outliner);
diff --git a/source/blender/makesdna/DNA_outliner_types.h b/source/blender/makesdna/DNA_outliner_types.h
index 16129768b60..7a39e0caef3 100644
--- a/source/blender/makesdna/DNA_outliner_types.h
+++ b/source/blender/makesdna/DNA_outliner_types.h
@@ -73,6 +73,16 @@ enum {
 
 /** #TreeStoreElem.types */
 typedef enum eTreeStoreElemType {
+  /**
+   * If an element is of this type, `TreeStoreElem.id` points to a valid ID and the ID-type can be
+   * received through `TreeElement.idcode` (or `GS(TreeStoreElem.id->name)`). Note however that the
+   * types below may also have a valid ID pointer (see #TSE_IS_REAL_ID()).
+   *
+   * In cases where the type is still checked against "0" (even implicitly), please replace it with
+   * an explicit check against `TSE_SOME_ID`.
+   */
+  TSE_SOME_ID = 0,
+
   TSE_NLA = 1, /* NO ID */
   TSE_NLA_ACTION = 2,
   TSE_DEFGROUP_BASE = 3,