source/blender/blenkernel/BKE_customdata.h

@@ -428,7 +428,7 @@ void CustomData_swap_corners(struct CustomData *data, int index, const int *corn
 /**
  * Swap two items of given custom data, in all available layers.
  */
-void CustomData_swap(struct CustomData *data, int index_a, int index_b);
+void CustomData_swap(struct CustomData *data, int index_a, int index_b, const int totelem);
 
 /**
  * Retrieve a pointer to an element of the active layer of the given \a type, chosen by the

source/blender/blenkernel/BKE_mesh.h

@@ -581,6 +581,7 @@ void BKE_mesh_polygon_flip_ex(int poly_offset,
                               int *corner_verts,
                               int *corner_edges,
                               struct CustomData *ldata,
+                              int tot_loop,
                               float (*lnors)[3],
                               struct MDisps *mdisp,
                               bool use_loop_mdisp_flip);
@@ -600,6 +601,7 @@ void BKE_mesh_polys_flip(const int *poly_offsets,
                          int *corner_verts,
                          int *corner_edges,
                          struct CustomData *ldata,
+                         int totloop,
                          int totpoly);
 
 /**

source/blender/blenkernel/intern/customdata.cc

@@ -3505,7 +3505,7 @@ void CustomData_swap_corners(CustomData *data, const int index, const int *corne
   }
 }
 
-void CustomData_swap(CustomData *data, const int index_a, const int index_b)
+void CustomData_swap(CustomData *data, const int index_a, const int index_b, const int totelem)
 {
   char buff_static[256];
 
@@ -3514,17 +3514,17 @@ void CustomData_swap(CustomData *data, const int index_a, const int index_b)
   }
 
   for (int i = 0; i < data->totlayer; i++) {
-    const LayerTypeInfo *typeInfo = layerType_getInfo(eCustomDataType(data->layers[i].type));
+    CustomDataLayer &layer = data->layers[i];
+    ensure_layer_data_is_mutable(layer, totelem);
+    const LayerTypeInfo *typeInfo = layerType_getInfo(eCustomDataType(layer.type));
     const size_t size = typeInfo->size;
     const size_t offset_a = size * index_a;
     const size_t offset_b = size * index_b;
 
     void *buff = size <= sizeof(buff_static) ? buff_static : MEM_mallocN(size, __func__);
-    memcpy(buff, POINTER_OFFSET(data->layers[i].data, offset_a), size);
-    memcpy(POINTER_OFFSET(data->layers[i].data, offset_a),
-           POINTER_OFFSET(data->layers[i].data, offset_b),
-           size);
-    memcpy(POINTER_OFFSET(data->layers[i].data, offset_b), buff, size);
+    memcpy(buff, POINTER_OFFSET(layer.data, offset_a), size);
+    memcpy(POINTER_OFFSET(layer.data, offset_a), POINTER_OFFSET(layer.data, offset_b), size);
+    memcpy(POINTER_OFFSET(layer.data, offset_b), buff, size);
 
     if (buff != buff_static) {
       MEM_freeN(buff);

source/blender/blenkernel/intern/mesh_evaluate.cc

@@ -539,6 +539,7 @@ void BKE_mesh_polygon_flip_ex(const int poly_offset,
                               int *corner_verts,
                               int *corner_edges,
                               CustomData *ldata,
+                              int tot_loop,
                               float (*lnors)[3],
                               MDisps *mdisp,
                               const bool use_loop_mdisp_flip)
@@ -577,7 +578,7 @@ void BKE_mesh_polygon_flip_ex(const int poly_offset,
     if (lnors) {
       swap_v3_v3(lnors[loopstart], lnors[loopend]);
     }
-    CustomData_swap(ldata, loopstart, loopend);
+    CustomData_swap(ldata, loopstart, loopend, tot_loop);
   }
   /* Even if we did not swap the other 'pivot' loop, we need to set its swapped edge. */
   if (loopstart == loopend) {
@@ -594,11 +595,15 @@ void BKE_mesh_polygon_flip(const int poly_offset,
 {
   MDisps *mdisp = (MDisps *)CustomData_get_layer_for_write(ldata, CD_MDISPS, totloop);
   BKE_mesh_polygon_flip_ex(
-      poly_offset, poly_size, corner_verts, corner_edges, ldata, nullptr, mdisp, true);
+      poly_offset, poly_size, corner_verts, corner_edges, ldata, totloop, nullptr, mdisp, true);
 }
 
-void BKE_mesh_polys_flip(
-    const int *poly_offsets, int *corner_verts, int *corner_edges, CustomData *ldata, int totpoly)
+void BKE_mesh_polys_flip(const int *poly_offsets,
+                         int *corner_verts,
+                         int *corner_edges,
+                         CustomData *ldata,
+                         int totloop,
+                         int totpoly)
 {
   const blender::OffsetIndices polys(blender::Span(poly_offsets, totpoly + 1));
   MDisps *mdisp = (MDisps *)CustomData_get_layer_for_write(ldata, CD_MDISPS, totpoly);
@@ -608,6 +613,7 @@ void BKE_mesh_polys_flip(
                              corner_verts,
                              corner_edges,
                              ldata,
+                             totloop,
                              nullptr,
                              mdisp,
                              true);

source/blender/makesrna/intern/rna_mesh_api.c

@@ -175,6 +175,7 @@ static void rna_Mesh_flip_normals(Mesh *mesh)
                       BKE_mesh_corner_verts_for_write(mesh),
                       BKE_mesh_corner_edges_for_write(mesh),
                       &mesh->ldata,
+                      mesh->totloop,
                       mesh->totpoly);
   BKE_mesh_tessface_clear(mesh);
   BKE_mesh_runtime_clear_geometry(mesh);

source/blender/modifiers/intern/MOD_normal_edit.cc

@@ -209,6 +209,7 @@ static bool polygons_check_flip(blender::MutableSpan<int> corner_verts,
                                corner_verts.data(),
                                corner_edges.data(),
                                ldata,
+                               corner_edges.size(),
                                reinterpret_cast<float(*)[3]>(nos),
                                mdisp,
                                true);