Fix T84420: Linking regular materials to gpencil

When using "Make Links"->"Materials" regular materials could be linked
onto grease pencil objects. This caused a number of issues.

The fix changes the `allow_make_links_data` function to make sure that
if one object is of type `OB_GPENCIL`, the other has to be aswell.

Reviewed By: antoniov

Maniphest Tasks: T84420

Differential Revision: https://developer.blender.org/D10014

1 files changed, 5 insertions, 1 deletions

diff --git a/source/blender/editors/object/object_relations.c b/source/blender/editors/object/object_relations.c
index ee9ef192d18..acad1b43cbb 100644
--- a/source/blender/editors/object/object_relations.c
+++ b/source/blender/editors/object/object_relations.c
@@ -1567,7 +1567,11 @@ static bool allow_make_links_data(const int type, Object *ob_src, Object *ob_dst
       }
       break;
     case MAKE_LINKS_MATERIALS:
-      if (OB_TYPE_SUPPORT_MATERIAL(ob_src->type) && OB_TYPE_SUPPORT_MATERIAL(ob_dst->type)) {
+      if (OB_TYPE_SUPPORT_MATERIAL(ob_src->type) && OB_TYPE_SUPPORT_MATERIAL(ob_dst->type) &&
+          /* Linking non-grease-pencil materials to a grease-pencil object causes issues.
+           * We make sure that if one of the objects is a grease-pencil object, the other must be
+           * as well. */
+          ((ob_src->type == OB_GPENCIL) == (ob_dst->type == OB_GPENCIL))) {
         return true;
       }
       break;