Fix T96367: Crash snapping to instances on an object

In rare cases the mesh has not been evaluated when snapping, this fix
just prevents the crash as is done elsewhere in Blender when the
evaluated mesh isn't available, there is a separate report (T96536)
about evaluation not working properly.

1 files changed, 13 insertions, 2 deletions

diff --git a/source/blender/editors/transform/transform_snap_object.cc b/source/blender/editors/transform/transform_snap_object.cc
index fade7f47d9c..0505772c668 100644
--- a/source/blender/editors/transform/transform_snap_object.cc
+++ b/source/blender/editors/transform/transform_snap_object.cc
@@ -168,8 +168,13 @@ struct SnapObjectContext {
 /** \name Utilities
  * \{ */
 
-/* Mesh used for snapping.
- * If nullptr the BMesh should be used. */
+/**
+ * Mesh used for snapping.
+ *
+ * - When the return value is null the `BKE_editmesh_from_object(ob_eval)` should be used.
+ * - In rare cases there is no evaluated mesh available and a null result doesn't imply an
+ *   edit-mesh, so callers need to account for a null edit-mesh too, see: T96536.
+ */
 static const Mesh *mesh_for_snap(Object *ob_eval, eSnapEditType edit_mode_type, bool *r_use_hide)
 {
   const Mesh *me_eval = BKE_object_get_evaluated_mesh(ob_eval);
@@ -998,6 +1003,9 @@ static void raycast_obj_fn(SnapObjectContext *sctx,
       const Mesh *me_eval = mesh_for_snap(ob_eval, edit_mode_type, &use_hide);
       if (me_eval == nullptr) {
         BMEditMesh *em = BKE_editmesh_from_object(ob_eval);
+        if (UNLIKELY(!em)) { /* See #mesh_for_snap doc-string. */
+          return;
+        }
         BLI_assert_msg(em == BKE_editmesh_from_object(DEG_get_original_object(ob_eval)),
                        "Make sure there is only one pointer for looptris");
         retval = raycastEditMesh(sctx,
@@ -2696,6 +2704,9 @@ static void snap_obj_fn(SnapObjectContext *sctx,
       const Mesh *me_eval = mesh_for_snap(ob_eval, edit_mode_type, &use_hide);
       if (me_eval == nullptr) {
         BMEditMesh *em = BKE_editmesh_from_object(ob_eval);
+        if (UNLIKELY(!em)) { /* See #mesh_for_snap doc-string. */
+          return;
+        }
         BLI_assert_msg(em == BKE_editmesh_from_object(DEG_get_original_object(ob_eval)),
                        "Make sure there is only one pointer for looptris");
         retval = snapEditMesh(