Allow derived coord references to be lazy. (#6517)

* Allow derived coord references to be lazy, when the source AuxCoord is.

* Small doctest fix.

* Add whatsnew.

Author: pp-mo

docs/src/userguide/cube_statistics.rst

@@ -143,7 +143,7 @@ These areas can now be passed to the ``collapsed`` method as weights:
             forecast_reference_time     2009-11-19 04:00:00
             grid_latitude               1.5145501 degrees, bound=(0.13755022, 2.89155) degrees
             grid_longitude              358.74948 degrees, bound=(357.48724, 360.01172) degrees
-            surface_altitude            399.625 m, bound=(-14.0, 813.25) m
+            surface_altitude            <lazy>+bound
         Cell methods:
             0                           grid_longitude: grid_latitude: mean
         Attributes:
@@ -191,7 +191,7 @@ the units of the resulting cube are multiplied by an area unit:
             forecast_reference_time     2009-11-19 04:00:00
             grid_latitude               1.5145501 degrees, bound=(0.13755022, 2.89155) degrees
             grid_longitude              358.74948 degrees, bound=(357.48724, 360.01172) degrees
-            surface_altitude            399.625 m, bound=(-14.0, 813.25) m
+            surface_altitude            <lazy>+bound
         Cell methods:
             0                           grid_longitude: grid_latitude: sum
         Attributes:

docs/src/whatsnew/latest.rst

@@ -46,6 +46,9 @@ This document explains the changes made to Iris for this release
    the behaviour when calling :attr:`iris.MeshCoord.points` and :attr:`MeshCoord.bounds`, which will return
    real data but will leave the :class:`iris.MeshCoord` (and attached mesh) lazy. (:issue:`4757`, :pull:`6405`)
 
+#. `@pp-mo`_ made it possible for the reference surfaces of derived coordinates, like orography, to be lazy.
+   (:pull: 6517).
+
 
 🐛 Bugs Fixed
 =============

lib/iris/fileformats/rules.py

@@ -159,7 +159,7 @@ def _dereference_args(factory, reference_targets, regrid_cache, cube):
                 src, cube = _ensure_aligned(regrid_cache, src, cube)
                 if src is not None:
                     new_coord = iris.coords.AuxCoord(
-                        src.data,
+                        src.core_data(),
                         src.standard_name,
                         src.long_name,
                         src.var_name,