[FIX] stock: extra move in return

- Create a PO with 5 units of A and 10 units of B
- Receive the products
- Return the picking, but return 12 units of A and 10 units of B

The first picking (done state) will contain:
- 7 units of A, 0 qty done => it is an extra move
- 10 units of B, 10 qty done

But another picking (not done) is created, with
- 5 units of A, 12 qty done

The core of the issue is that the `origin_returned_move_id` is not
copied on the extra move. Therefore, it is not merged by `_merge_moves`
since the latter field is in the list of
`_prepare_merge_moves_distinct_fields`.

opw-1882959

addons/stock/models/stock_move.py

@@ -1031,6 +1031,7 @@ class StockMove(models.Model):
 
     def _prepare_extra_move_vals(self, qty):
         vals = {
+            'origin_returned_move_id': self.origin_returned_move_id.id,
             'product_uom_qty': qty,
             'picking_id': self.picking_id.id,
             'price_unit': self.price_unit,

addons/stock/tests/test_move2.py

@@ -252,6 +252,30 @@ class TestPickShip(TestStockCommon):
             "Downstream computation from original picking worked"
         )
 
+    def test_mto_moves_return_extra(self):
+        picking_pick, picking_client = self.create_pick_ship()
+        stock_location = self.env['stock.location'].browse(self.stock_location)
+        self.env['stock.quant']._update_available_quantity(self.productA, stock_location, 10.0)
+
+        picking_pick.action_assign()
+        picking_pick.move_lines[0].move_line_ids[0].qty_done = 10.0
+        picking_pick.action_done()
+        self.assertEqual(picking_pick.state, 'done')
+        self.assertEqual(picking_client.state, 'assigned')
+
+        # return more than we've done
+        stock_return_picking = self.env['stock.return.picking']\
+            .with_context(active_ids=picking_pick.ids, active_id=picking_pick.ids[0])\
+            .create({})
+        stock_return_picking.product_return_moves.quantity = 12.0 # Return 2 extra
+        stock_return_picking_action = stock_return_picking.create_returns()
+        return_pick = self.env['stock.picking'].browse(stock_return_picking_action['res_id'])
+
+        # Verify the extra move has been merged with the original move
+        self.assertAlmostEqual(return_pick.move_lines.product_uom_qty, 12.0)
+        self.assertAlmostEqual(return_pick.move_lines.quantity_done, 0.0)
+        self.assertAlmostEqual(return_pick.move_lines.reserved_availability, 10.0)
+
     def test_mto_resupply_cancel_ship(self):
         """ This test simulates a pick pack ship with a resupply route
         set. Pick and pack are validated, ship is cancelled. This test