UI/Nodes: Improve feedback when adding node fails (e.g. on drag & drop)

This is especially useful when trying to add a node group instance, e.g. via
drag & drop from the Outliner or Asset Browser.
Previously this would just silently fail, with no information why. This is a
source of confusion, e.g. earlier, it took me a moment to realize I was
dragging a node group into itself, which failed of course.
Blender should always try to help the user with useful error messages.

Adds error messages like: "Nesting a node group inside of itself is not
allowed", "Not a compositor node tree", etc.

Adds a disabled hint return argument to node and node tree polling functions.
On error the hint is reported, or could even be shown in advance (e.g. if
checked via an operator poll option).

Differential Revision: https://developer.blender.org/D10422

Reviewed by: Jacques Lucke

21 files changed, 224 insertions, 79 deletions

diff --git a/source/blender/blenkernel/BKE_node.h b/source/blender/blenkernel/BKE_node.h
index 30c76dc894c..d6c4ad037e2 100644
--- a/source/blender/blenkernel/BKE_node.h
+++ b/source/blender/blenkernel/BKE_node.h
@@ -289,10 +289,22 @@ typedef struct bNodeType {
   void (*freefunc_api)(struct PointerRNA *ptr);
   void (*copyfunc_api)(struct PointerRNA *ptr, const struct bNode *src_node);
 
-  /* can this node type be added to a node tree */
-  bool (*poll)(struct bNodeType *ntype, struct bNodeTree *nodetree);
-  /* can this node be added to a node tree */
-  bool (*poll_instance)(struct bNode *node, struct bNodeTree *nodetree);
+  /**
+   * Can this node type be added to a node tree?
+   * \param r_disabled_hint: Optional hint to display in the UI when the poll fails.
+   *                         The callback can set this to a static string without having to
+   *                         null-check it (or without setting it to null if it's not used).
+   *                         The caller must pass a valid `const char **` and null-initialize it
+   *                         when it's not just a dummy, that is, if it actually wants to access
+   *                         the returned disabled-hint (null-check needed!).
+   */
+  bool (*poll)(struct bNodeType *ntype, struct bNodeTree *nodetree, const char **r_disabled_hint);
+  /** Can this node be added to a node tree?
+   * \param r_disabled_hint: See `poll()`.
+   */
+  bool (*poll_instance)(struct bNode *node,
+                        struct bNodeTree *nodetree,
+                        const char **r_disabled_hint);
 
   /* optional handling of link insertion */
   void (*insert_link)(struct bNodeTree *ntree, struct bNode *node, struct bNodeLink *link);
@@ -804,7 +816,9 @@ void BKE_node_preview_set_pixel(
 void nodeLabel(struct bNodeTree *ntree, struct bNode *node, char *label, int maxlen);
 const char *nodeSocketLabel(const struct bNodeSocket *sock);
 
-int nodeGroupPoll(struct bNodeTree *nodetree, struct bNodeTree *grouptree);
+bool nodeGroupPoll(struct bNodeTree *nodetree,
+                   struct bNodeTree *grouptree,
+                   const char **r_disabled_hint);
 
 /* Init a new node type struct with default values and callbacks */
 void node_type_base(struct bNodeType *ntype, int type, const char *name, short nclass, short flag);
diff --git a/source/blender/blenkernel/intern/node.cc b/source/blender/blenkernel/intern/node.cc
index 52f6c4f93ad..02195e0d60f 100644
--- a/source/blender/blenkernel/intern/node.cc
+++ b/source/blender/blenkernel/intern/node.cc
@@ -2007,7 +2007,8 @@ bNode *nodeAddStaticNode(const struct bContext *C, bNodeTree *ntree, int type)
     /* do an extra poll here, because some int types are used
      * for multiple node types, this helps find the desired type
      */
-    if (ntype->type == type && (!ntype->poll || ntype->poll(ntype, ntree))) {
+    const char *disabled_hint;
+    if (ntype->type == type && (!ntype->poll || ntype->poll(ntype, ntree, &disabled_hint))) {
       idname = ntype->idname;
       break;
     }
@@ -4407,15 +4408,17 @@ static void node_type_base_defaults(bNodeType *ntype)
 }
 
 /* allow this node for any tree type */
-static bool node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *UNUSED(ntree))
+static bool node_poll_default(bNodeType *UNUSED(ntype),
+                              bNodeTree *UNUSED(ntree),
+                              const char **UNUSED(disabled_hint))
 {
   return true;
 }
 
 /* use the basic poll function */
-static bool node_poll_instance_default(bNode *node, bNodeTree *ntree)
+static bool node_poll_instance_default(bNode *node, bNodeTree *ntree, const char **disabled_hint)
 {
-  return node->typeinfo->poll(node->typeinfo, ntree);
+  return node->typeinfo->poll(node->typeinfo, ntree, disabled_hint);
 }
 
 /* NOLINTNEXTLINE: readability-function-size */
@@ -4634,7 +4637,9 @@ void node_type_internal_links(bNodeType *ntype,
 
 /* callbacks for undefined types */
 
-static bool node_undefined_poll(bNodeType *UNUSED(ntype), bNodeTree *UNUSED(nodetree))
+static bool node_undefined_poll(bNodeType *UNUSED(ntype),
+                                bNodeTree *UNUSED(nodetree),
+                                const char **UNUSED(r_disabled_hint))
 {
   /* this type can not be added deliberately, it's just a placeholder */
   return false;
diff --git a/source/blender/editors/space_node/node_add.c b/source/blender/editors/space_node/node_add.c
index 82a315366dc..a28d467df2e 100644
--- a/source/blender/editors/space_node/node_add.c
+++ b/source/blender/editors/space_node/node_add.c
@@ -339,7 +339,25 @@ static bNodeTree *node_add_group_get_and_poll_group_node_tree(Main *bmain,
   if (!node_group) {
     return NULL;
   }
-  if ((node_group->type != ntree->type) || !nodeGroupPoll(ntree, node_group)) {
+
+  const char *disabled_hint = NULL;
+  if ((node_group->type != ntree->type) || !nodeGroupPoll(ntree, node_group, &disabled_hint)) {
+    if (disabled_hint) {
+      BKE_reportf(op->reports,
+                  RPT_ERROR,
+                  "Can not add node group '%s' to '%s':\n  %s",
+                  node_group->id.name + 2,
+                  ntree->id.name + 2,
+                  disabled_hint);
+    }
+    else {
+      BKE_reportf(op->reports,
+                  RPT_ERROR,
+                  "Can not add node group '%s' to '%s'",
+                  node_group->id.name + 2,
+                  ntree->id.name + 2);
+    }
+
     return NULL;
   }
 
diff --git a/source/blender/editors/space_node/node_edit.c b/source/blender/editors/space_node/node_edit.c
index 518f5639c93..1cbd0fd607c 100644
--- a/source/blender/editors/space_node/node_edit.c
+++ b/source/blender/editors/space_node/node_edit.c
@@ -2241,13 +2241,25 @@ static int node_clipboard_paste_exec(bContext *C, wmOperator *op)
   /* make sure all clipboard nodes would be valid in the target tree */
   bool all_nodes_valid = true;
   LISTBASE_FOREACH (bNode *, node, clipboard_nodes_lb) {
-    if (!node->typeinfo->poll_instance || !node->typeinfo->poll_instance(node, ntree)) {
+    const char *disabled_hint = NULL;
+    if (!node->typeinfo->poll_instance ||
+        !node->typeinfo->poll_instance(node, ntree, &disabled_hint)) {
       all_nodes_valid = false;
-      BKE_reportf(op->reports,
-                  RPT_ERROR,
-                  "Cannot add node %s into node tree %s",
-                  node->name,
-                  ntree->id.name + 2);
+      if (disabled_hint) {
+        BKE_reportf(op->reports,
+                    RPT_ERROR,
+                    "Cannot add node %s into node tree %s:\n  %s",
+                    node->name,
+                    ntree->id.name + 2,
+                    disabled_hint);
+      }
+      else {
+        BKE_reportf(op->reports,
+                    RPT_ERROR,
+                    "Cannot add node %s into node tree %s",
+                    node->name,
+                    ntree->id.name + 2);
+      }
     }
   }
   if (!all_nodes_valid) {
diff --git a/source/blender/editors/space_node/node_group.c b/source/blender/editors/space_node/node_group.c
index e1de4bfc21e..335e2f93ff3 100644
--- a/source/blender/editors/space_node/node_group.c
+++ b/source/blender/editors/space_node/node_group.c
@@ -679,8 +679,19 @@ static bool node_group_make_test_selected(bNodeTree *ntree,
   /* check poll functions for selected nodes */
   LISTBASE_FOREACH (bNode *, node, &ntree->nodes) {
     if (node_group_make_use_node(node, gnode)) {
-      if (node->typeinfo->poll_instance && !node->typeinfo->poll_instance(node, ngroup)) {
-        BKE_reportf(reports, RPT_WARNING, "Can not add node '%s' in a group", node->name);
+      const char *disabled_hint = NULL;
+      if (node->typeinfo->poll_instance &&
+          !node->typeinfo->poll_instance(node, ngroup, &disabled_hint)) {
+        if (disabled_hint) {
+          BKE_reportf(reports,
+                      RPT_WARNING,
+                      "Can not add node '%s' in a group:\n  %s",
+                      node->name,
+                      disabled_hint);
+        }
+        else {
+          BKE_reportf(reports, RPT_WARNING, "Can not add node '%s' in a group", node->name);
+        }
         ok = false;
         break;
       }
diff --git a/source/blender/editors/space_node/node_templates.c b/source/blender/editors/space_node/node_templates.c
index 3873985d93a..54145f62895 100644
--- a/source/blender/editors/space_node/node_templates.c
+++ b/source/blender/editors/space_node/node_templates.c
@@ -330,7 +330,9 @@ static void ui_node_link_items(NodeLinkArg *arg,
     int i;
 
     for (ngroup = arg->bmain->nodetrees.first; ngroup; ngroup = ngroup->id.next) {
-      if ((ngroup->type != arg->ntree->type) || !nodeGroupPoll(arg->ntree, ngroup)) {
+      const char *disabled_hint;
+      if ((ngroup->type != arg->ntree->type) ||
+          !nodeGroupPoll(arg->ntree, ngroup, &disabled_hint)) {
         continue;
       }
 
@@ -343,7 +345,9 @@ static void ui_node_link_items(NodeLinkArg *arg,
 
       i = 0;
       for (ngroup = arg->bmain->nodetrees.first; ngroup; ngroup = ngroup->id.next) {
-        if ((ngroup->type != arg->ntree->type) || !nodeGroupPoll(arg->ntree, ngroup)) {
+        const char *disabled_hint;
+        if ((ngroup->type != arg->ntree->type) ||
+            !nodeGroupPoll(arg->ntree, ngroup, &disabled_hint)) {
           continue;
         }
 
@@ -481,7 +485,8 @@ static void ui_node_menu_column(NodeLinkArg *arg, int nclass, const char *cname)
   BLI_array_declare(sorted_ntypes);
 
   NODE_TYPES_BEGIN (ntype) {
-    if (!(ntype->poll && ntype->poll(ntype, ntree))) {
+    const char *disabled_hint;
+    if (!(ntype->poll && ntype->poll(ntype, ntree, &disabled_hint))) {
       continue;
     }
 
diff --git a/source/blender/makesrna/intern/rna_nodetree.c b/source/blender/makesrna/intern/rna_nodetree.c
index b79381ac26f..1016d31f11b 100644
--- a/source/blender/makesrna/intern/rna_nodetree.c
+++ b/source/blender/makesrna/intern/rna_nodetree.c
@@ -1085,13 +1085,25 @@ static bNode *rna_NodeTree_node_new(bNodeTree *ntree,
     return NULL;
   }
 
-  if (ntype->poll && !ntype->poll(ntype, ntree)) {
-    BKE_reportf(reports,
-                RPT_ERROR,
-                "Cannot add node of type %s to node tree '%s'",
-                type,
-                ntree->id.name + 2);
-    return NULL;
+  const char *disabled_hint = NULL;
+  if (ntype->poll && !ntype->poll(ntype, ntree, &disabled_hint)) {
+    if (disabled_hint) {
+      BKE_reportf(reports,
+                  RPT_ERROR,
+                  "Cannot add node of type %s to node tree '%s'\n  %s",
+                  type,
+                  ntree->id.name + 2,
+                  disabled_hint);
+      return NULL;
+    }
+    else {
+      BKE_reportf(reports,
+                  RPT_ERROR,
+                  "Cannot add node of type %s to node tree '%s'",
+                  type,
+                  ntree->id.name + 2);
+      return NULL;
+    }
   }
 
   node = nodeAddNode(C, ntree, type);
@@ -1531,7 +1543,7 @@ char *rna_Node_ImageUser_path(PointerRNA *ptr)
   return NULL;
 }
 
-static bool rna_Node_poll(bNodeType *ntype, bNodeTree *ntree)
+static bool rna_Node_poll(bNodeType *ntype, bNodeTree *ntree, const char **UNUSED(r_disabled_hint))
 {
   extern FunctionRNA rna_Node_poll_func;
 
@@ -1556,7 +1568,9 @@ static bool rna_Node_poll(bNodeType *ntype, bNodeTree *ntree)
   return visible;
 }
 
-static bool rna_Node_poll_instance(bNode *node, bNodeTree *ntree)
+static bool rna_Node_poll_instance(bNode *node,
+                                   bNodeTree *ntree,
+                                   const char **UNUSED(disabled_info))
 {
   extern FunctionRNA rna_Node_poll_instance_func;
 
@@ -1581,10 +1595,12 @@ static bool rna_Node_poll_instance(bNode *node, bNodeTree *ntree)
   return visible;
 }
 
-static bool rna_Node_poll_instance_default(bNode *node, bNodeTree *ntree)
+static bool rna_Node_poll_instance_default(bNode *node,
+                                           bNodeTree *ntree,
+                                           const char **disabled_info)
 {
   /* use the basic poll function */
-  return rna_Node_poll(node->typeinfo, ntree);
+  return rna_Node_poll(node->typeinfo, ntree, disabled_info);
 }
 
 static void rna_Node_update_reg(bNodeTree *ntree, bNode *node)
@@ -3214,18 +3230,20 @@ static PointerRNA rna_NodeInternal_output_template(StructRNA *srna, int index)
 static bool rna_NodeInternal_poll(StructRNA *srna, bNodeTree *ntree)
 {
   bNodeType *ntype = RNA_struct_blender_type_get(srna);
-  return ntype && (!ntype->poll || ntype->poll(ntype, ntree));
+  const char *disabled_hint;
+  return ntype && (!ntype->poll || ntype->poll(ntype, ntree, &disabled_hint));
 }
 
 static bool rna_NodeInternal_poll_instance(bNode *node, bNodeTree *ntree)
 {
   bNodeType *ntype = node->typeinfo;
+  const char *disabled_hint;
   if (ntype->poll_instance) {
-    return ntype->poll_instance(node, ntree);
+    return ntype->poll_instance(node, ntree, &disabled_hint);
   }
   else {
     /* fall back to basic poll function */
-    return !ntype->poll || ntype->poll(ntype, ntree);
+    return !ntype->poll || ntype->poll(ntype, ntree, &disabled_hint);
   }
 }
 
@@ -3408,7 +3426,8 @@ static void rna_NodeGroup_node_tree_set(PointerRNA *ptr,
   bNode *node = ptr->data;
   bNodeTree *ngroup = value.data;
 
-  if (nodeGroupPoll(ntree, ngroup)) {
+  const char *disabled_hint = NULL;
+  if (nodeGroupPoll(ntree, ngroup, &disabled_hint)) {
     if (node->id) {
       id_us_min(node->id);
     }
@@ -3430,7 +3449,8 @@ static bool rna_NodeGroup_node_tree_poll(PointerRNA *ptr, const PointerRNA value
     return false;
   }
 
-  return nodeGroupPoll(ntree, ngroup);
+  const char *disabled_hint = NULL;
+  return nodeGroupPoll(ntree, ngroup, &disabled_hint);
 }
 
 static StructRNA *rna_NodeGroup_interface_typef(PointerRNA *ptr)
diff --git a/source/blender/nodes/composite/node_composite_util.c b/source/blender/nodes/composite/node_composite_util.c
index b6cbffea413..6cc17d8c272 100644
--- a/source/blender/nodes/composite/node_composite_util.c
+++ b/source/blender/nodes/composite/node_composite_util.c
@@ -23,9 +23,15 @@
 
 #include "node_composite_util.h"
 
-bool cmp_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+bool cmp_node_poll_default(bNodeType *UNUSED(ntype),
+                           bNodeTree *ntree,
+                           const char **r_disabled_hint)
 {
-  return STREQ(ntree->idname, "CompositorNodeTree");
+  if (!STREQ(ntree->idname, "CompositorNodeTree")) {
+    *r_disabled_hint = "Not a compositor node tree";
+    return false;
+  }
+  return true;
 }
 
 void cmp_node_update_default(bNodeTree *UNUSED(ntree), bNode *node)
diff --git a/source/blender/nodes/composite/node_composite_util.h b/source/blender/nodes/composite/node_composite_util.h
index 800c55df4d6..4fcccbb79f0 100644
--- a/source/blender/nodes/composite/node_composite_util.h
+++ b/source/blender/nodes/composite/node_composite_util.h
@@ -54,7 +54,9 @@ extern "C" {
 
 #define CMP_SCALE_MAX 12000
 
-bool cmp_node_poll_default(struct bNodeType *ntype, struct bNodeTree *ntree);
+bool cmp_node_poll_default(struct bNodeType *ntype,
+                           struct bNodeTree *ntree,
+                           const char **r_disabled_info);
 void cmp_node_update_default(struct bNodeTree *ntree, struct bNode *node);
 void cmp_node_type_base(
     struct bNodeType *ntype, int type, const char *name, short nclass, short flag);
diff --git a/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc b/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc
index c5135482ec2..d9b36924516 100644
--- a/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc
+++ b/source/blender/nodes/composite/nodes/node_composite_cryptomatte.cc
@@ -263,7 +263,9 @@ static void node_copy_cryptomatte(bNodeTree *UNUSED(dest_ntree),
   dest_node->storage = dest_nc;
 }
 
-static bool node_poll_cryptomatte(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+static bool node_poll_cryptomatte(bNodeType *UNUSED(ntype),
+                                  bNodeTree *ntree,
+                                  const char **r_disabled_hint)
 {
   if (STREQ(ntree->idname, "CompositorNodeTree")) {
     Scene *scene;
@@ -276,8 +278,13 @@ static bool node_poll_cryptomatte(bNodeType *UNUSED(ntype), bNodeTree *ntree)
       }
     }
 
+    if (scene == nullptr) {
+      *r_disabled_hint =
+          "The node tree must be the compositing node tree of any scene in the file";
+    }
     return scene != nullptr;
   }
+  *r_disabled_hint = "Not a compositor node tree";
   return false;
 }
 
diff --git a/source/blender/nodes/composite/nodes/node_composite_image.c b/source/blender/nodes/composite/nodes/node_composite_image.c
index 53ea02ff8a7..243300b0a44 100644
--- a/source/blender/nodes/composite/nodes/node_composite_image.c
+++ b/source/blender/nodes/composite/nodes/node_composite_image.c
@@ -526,24 +526,32 @@ static void node_composit_init_rlayers(const bContext *C, PointerRNA *ptr)
   }
 }
 
-static bool node_composit_poll_rlayers(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+static bool node_composit_poll_rlayers(bNodeType *UNUSED(ntype),
+                                       bNodeTree *ntree,
+                                       const char **r_disabled_hint)
 {
-  if (STREQ(ntree->idname, "CompositorNodeTree")) {
-    Scene *scene;
+  if (!STREQ(ntree->idname, "CompositorNodeTree")) {
+    *r_disabled_hint = "Not a compositor node tree";
+    return false;
+  }
 
-    /* XXX ugly: check if ntree is a local scene node tree.
-     * Render layers node can only be used in local scene->nodetree,
-     * since it directly links to the scene.
-     */
-    for (scene = G.main->scenes.first; scene; scene = scene->id.next) {
-      if (scene->nodetree == ntree) {
-        break;
-      }
+  Scene *scene;
+
+  /* XXX ugly: check if ntree is a local scene node tree.
+   * Render layers node can only be used in local scene->nodetree,
+   * since it directly links to the scene.
+   */
+  for (scene = G.main->scenes.first; scene; scene = scene->id.next) {
+    if (scene->nodetree == ntree) {
+      break;
     }
+  }
 
-    return (scene != NULL);
+  if (scene == NULL) {
+    *r_disabled_hint = "The node tree must be the compositing node tree of any scene in the file";
+    return false;
   }
-  return false;
+  return true;
 }
 
 static void node_composit_free_rlayers(bNode *node)
diff --git a/source/blender/nodes/function/node_function_util.cc b/source/blender/nodes/function/node_function_util.cc
index 827572d1069..8ff8b416310 100644
--- a/source/blender/nodes/function/node_function_util.cc
+++ b/source/blender/nodes/function/node_function_util.cc
@@ -17,10 +17,16 @@
 #include "node_function_util.hh"
 #include "node_util.h"
 
-bool fn_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+static bool fn_node_poll_default(bNodeType *UNUSED(ntype),
+                                 bNodeTree *ntree,
+                                 const char **r_disabled_hint)
 {
   /* Function nodes are only supported in simulation node trees so far. */
-  return STREQ(ntree->idname, "GeometryNodeTree");
+  if (!STREQ(ntree->idname, "GeometryNodeTree")) {
+    *r_disabled_hint = "Not a geometry node tree";
+    return false;
+  }
+  return true;
 }
 
 void fn_node_type_base(bNodeType *ntype, int type, const char *name, short nclass, short flag)
diff --git a/source/blender/nodes/function/node_function_util.hh b/source/blender/nodes/function/node_function_util.hh
index d57d1383019..9fbd6712827 100644
--- a/source/blender/nodes/function/node_function_util.hh
+++ b/source/blender/nodes/function/node_function_util.hh
@@ -38,4 +38,3 @@
 
 void fn_node_type_base(
     struct bNodeType *ntype, int type, const char *name, short nclass, short flag);
-bool fn_node_poll_default(struct bNodeType *ntype, struct bNodeTree *ntree);
diff --git a/source/blender/nodes/geometry/node_geometry_util.cc b/source/blender/nodes/geometry/node_geometry_util.cc
index 0f725ecf211..93cada2982b 100644
--- a/source/blender/nodes/geometry/node_geometry_util.cc
+++ b/source/blender/nodes/geometry/node_geometry_util.cc
@@ -56,9 +56,15 @@ void update_attribute_input_socket_availabilities(bNode &node,
 
 }  // namespace blender::nodes
 
-bool geo_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+bool geo_node_poll_default(bNodeType *UNUSED(ntype),
+                           bNodeTree *ntree,
+                           const char **r_disabled_hint)
 {
-  return STREQ(ntree->idname, "GeometryNodeTree");
+  if (!STREQ(ntree->idname, "GeometryNodeTree")) {
+    *r_disabled_hint = "Not a geometry node tree";
+    return false;
+  }
+  return true;
 }
 
 void geo_node_type_base(bNodeType *ntype, int type, const char *name, short nclass, short flag)
diff --git a/source/blender/nodes/geometry/node_geometry_util.hh b/source/blender/nodes/geometry/node_geometry_util.hh
index ad4f4331b6c..81092798ca1 100644
--- a/source/blender/nodes/geometry/node_geometry_util.hh
+++ b/source/blender/nodes/geometry/node_geometry_util.hh
@@ -36,7 +36,9 @@
 
 void geo_node_type_base(
     struct bNodeType *ntype, int type, const char *name, short nclass, short flag);
-bool geo_node_poll_default(struct bNodeType *ntype, struct bNodeTree *ntree);
+bool geo_node_poll_default(struct bNodeType *ntype,
+                           struct bNodeTree *ntree,
+                           const char **r_disabled_hint);
 
 namespace blender::nodes {
 void update_attribute_input_socket_availabilities(bNode &node,
diff --git a/source/blender/nodes/intern/node_common.c b/source/blender/nodes/intern/node_common.c
index 0b1ab85c059..7fc9f664df0 100644
--- a/source/blender/nodes/intern/node_common.c
+++ b/source/blender/nodes/intern/node_common.c
@@ -79,12 +79,12 @@ void node_group_label(bNodeTree *UNUSED(ntree), bNode *node, char *label, int ma
   BLI_strncpy(label, (node->id) ? node->id->name + 2 : IFACE_("Missing Data-Block"), maxlen);
 }
 
-bool node_group_poll_instance(bNode *node, bNodeTree *nodetree)
+bool node_group_poll_instance(bNode *node, bNodeTree *nodetree, const char **disabled_hint)
 {
-  if (node->typeinfo->poll(node->typeinfo, nodetree)) {
+  if (node->typeinfo->poll(node->typeinfo, nodetree, disabled_hint)) {
     bNodeTree *grouptree = (bNodeTree *)node->id;
     if (grouptree) {
-      return nodeGroupPoll(nodetree, grouptree);
+      return nodeGroupPoll(nodetree, grouptree, disabled_hint);
     }
 
     return true; /* without a linked node tree, group node is always ok */
@@ -93,25 +93,27 @@ bool node_group_poll_instance(bNode *node, bNodeTree *nodetree)
   return false;
 }
 
-int nodeGroupPoll(bNodeTree *nodetree, bNodeTree *grouptree)
+bool nodeGroupPoll(bNodeTree *nodetree, bNodeTree *grouptree, const char **r_disabled_hint)
 {
   bNode *node;
-  int valid = 1;
+  bool valid = true;
 
   /* unspecified node group, generally allowed
    * (if anything, should be avoided on operator level)
    */
   if (grouptree == NULL) {
-    return 1;
+    return true;
   }
 
   if (nodetree == grouptree) {
-    return 0;
+    *r_disabled_hint = "Nesting a node group inside of itself is not allowed";
+    return false;
   }
 
   for (node = grouptree->nodes.first; node; node = node->next) {
-    if (node->typeinfo->poll_instance && !node->typeinfo->poll_instance(node, nodetree)) {
-      valid = 0;
+    if (node->typeinfo->poll_instance &&
+        !node->typeinfo->poll_instance(node, nodetree, r_disabled_hint)) {
+      valid = false;
       break;
     }
   }
diff --git a/source/blender/nodes/intern/node_common.h b/source/blender/nodes/intern/node_common.h
index 7aad6782640..cdb7b6897b9 100644
--- a/source/blender/nodes/intern/node_common.h
+++ b/source/blender/nodes/intern/node_common.h
@@ -32,7 +32,9 @@ extern "C" {
 struct bNodeTree;
 
 void node_group_label(struct bNodeTree *ntree, struct bNode *node, char *label, int maxlen);
-bool node_group_poll_instance(struct bNode *node, struct bNodeTree *nodetree);
+bool node_group_poll_instance(struct bNode *node,
+                              struct bNodeTree *nodetree,
+                              const char **r_disabled_hint);
 
 void ntree_update_reroute_nodes(struct bNodeTree *ntree);
 
diff --git a/source/blender/nodes/shader/node_shader_util.c b/source/blender/nodes/shader/node_shader_util.c
index 1a2405e021f..04c32574a65 100644
--- a/source/blender/nodes/shader/node_shader_util.c
+++ b/source/blender/nodes/shader/node_shader_util.c
@@ -27,14 +27,24 @@
 
 #include "node_exec.h"
 
-bool sh_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+bool sh_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree, const char **r_disabled_hint)
 {
-  return STREQ(ntree->idname, "ShaderNodeTree");
+  if (!STREQ(ntree->idname, "ShaderNodeTree")) {
+    *r_disabled_hint = "Not a shader node tree";
+    return false;
+  }
+  return true;
 }
 
-static bool sh_fn_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+static bool sh_fn_poll_default(bNodeType *UNUSED(ntype),
+                               bNodeTree *ntree,
+                               const char **r_disabled_hint)
 {
-  return STREQ(ntree->idname, "ShaderNodeTree") || STREQ(ntree->idname, "GeometryNodeTree");
+  if (!STREQ(ntree->idname, "ShaderNodeTree") && !STREQ(ntree->idname, "GeometryNodeTree")) {
+    *r_disabled_hint = "Not a shader or geometry node tree";
+    return false;
+  }
+  return true;
 }
 
 void sh_node_type_base(
diff --git a/source/blender/nodes/shader/node_shader_util.h b/source/blender/nodes/shader/node_shader_util.h
index 91454c3c982..857a9914354 100644
--- a/source/blender/nodes/shader/node_shader_util.h
+++ b/source/blender/nodes/shader/node_shader_util.h
@@ -80,7 +80,9 @@
 extern "C" {
 #endif
 
-bool sh_node_poll_default(struct bNodeType *ntype, struct bNodeTree *ntree);
+bool sh_node_poll_default(struct bNodeType *ntype,
+                          struct bNodeTree *ntree,
+                          const char **r_disabled_hint);
 void sh_node_type_base(
     struct bNodeType *ntype, int type, const char *name, short nclass, short flag);
 void sh_fn_node_type_base(
diff --git a/source/blender/nodes/texture/node_texture_util.c b/source/blender/nodes/texture/node_texture_util.c
index 2091a8bf10e..570b10d6e89 100644
--- a/source/blender/nodes/texture/node_texture_util.c
+++ b/source/blender/nodes/texture/node_texture_util.c
@@ -39,9 +39,15 @@
 
 #include "node_texture_util.h"
 
-bool tex_node_poll_default(bNodeType *UNUSED(ntype), bNodeTree *ntree)
+bool tex_node_poll_default(bNodeType *UNUSED(ntype),
+                           bNodeTree *ntree,
+                           const char **r_disabled_hint)
 {
-  return STREQ(ntree->idname, "TextureNodeTree");
+  if (!STREQ(ntree->idname, "TextureNodeTree")) {
+    *r_disabled_hint = "Not a texture node tree";
+    return false;
+  }
+  return true;
 }
 
 void tex_node_type_base(
diff --git a/source/blender/nodes/texture/node_texture_util.h b/source/blender/nodes/texture/node_texture_util.h
index 74f27ef3974..8f63a1ad07d 100644
--- a/source/blender/nodes/texture/node_texture_util.h
+++ b/source/blender/nodes/texture/node_texture_util.h
@@ -106,7 +106,9 @@ typedef struct TexDelegate {
   int type;
 } TexDelegate;
 
-bool tex_node_poll_default(struct bNodeType *ntype, struct bNodeTree *ntree);
+bool tex_node_poll_default(struct bNodeType *ntype,
+                           struct bNodeTree *ntree,
+                           const char **r_disabled_hint);
 void tex_node_type_base(
     struct bNodeType *ntype, int type, const char *name, short nclass, short flag);