[FB] [PI-3478] Lenient cube arithmetic

.flake8

@@ -15,6 +15,8 @@ ignore =
     E402,
     # E501: line too long
     E501,
+    # E731: do not assign a lambda expression, use a def
+    E731,
     # W503: line break before binary operator
     W503,
     # W504: line break after binary operator

docs/iris/src/userguide/cube_maths.rst

@@ -60,6 +60,10 @@ but with the data representing their difference:
          Scalar coordinates:
               forecast_reference_time: 1859-09-01 06:00:00
               height: 1.5 m
+         Attributes:
+              Conventions: CF-1.5
+              Model scenario: E1
+              source: Data from Met Office Unified Model 6.05
 
 
 .. note::

lib/iris/_concatenate.py

@@ -331,11 +331,11 @@ def __init__(self, cube):
         axes = dict(T=0, Z=1, Y=2, X=3)
 
         # Coordinate sort function - by guessed coordinate axis, then
-        # by coordinate metadata, then by dimensions, in ascending order.
+        # by coordinate name, then by dimensions, in ascending order.
         def key_func(coord):
             return (
                 axes.get(guess_coord_axis(coord), len(axes) + 1),
-                coord.metadata,
+                coord.name(),
                 cube.coord_dims(coord),
             )
 
@@ -859,6 +859,9 @@ def _build_aux_coordinates(self):
                             points, bounds=bnds, **kwargs
                         )
                     except ValueError:
+                        # Ensure to remove the "circular" kwarg, which may be
+                        # present in the defn of a DimCoord being demoted.
+                        _ = kwargs.pop("circular", None)
                         coord = iris.coords.AuxCoord(
                             points, bounds=bnds, **kwargs
                         )