Fix concatenat_cube for multidimensional concatenation

docs/src/whatsnew/latest.rst

@@ -67,6 +67,9 @@ This document explains the changes made to Iris for this release
 #. `@stephenworsley`_ fixed incompatibilities with numpy v2.3 affecting arrays of dates and
    array printing. (:pull:`6518`)
 
+#. `@stephenworsley`_ fixed a bug which caused :meth:`~iris.cube.CubeList.concatenate_cube`
+   to fail when concatenating over multiple axes. (:pull:`6533`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/_concatenate.py

@@ -609,14 +609,15 @@ def add_coords(cube_signature: _CubeSignature, coord_type: str) -> None:
             add_coords(cube_signature, "ancillary_variables_and_dims")
 
     hashes = _compute_hashes(arrays)
+    msg = None
 
     # Register each cube with its appropriate proto-cube.
     for cube_signature in cube_signatures:
         registered = False
 
         # Register cube with an existing proto-cube.
         for proto_cube in proto_cubes:
-            registered = proto_cube.register(
+            registered, msg = proto_cube.register(
                 cube_signature,
                 hashes,
                 axis,
@@ -648,6 +649,11 @@ def _none_sort(proto_cube):
     count = len(concatenated_cubes)
     if count != 1 and count != len(cubes):
         concatenated_cubes = concatenate(concatenated_cubes)
+    elif msg is not None:
+        if error_on_mismatch:
+            raise iris.exceptions.ConcatenateError([msg])
+        else:
+            warnings.warn(msg, category=iris.warnings.IrisUserWarning)
 
     return concatenated_cubes
 
@@ -1115,7 +1121,7 @@ def register(
         check_cell_measures: bool = False,
         check_ancils: bool = False,
         check_derived_coords: bool = False,
-    ) -> bool:
+    ) -> tuple[bool, str | None]:
         """Determine if  the given source-cube is suitable for concatenation.
 
         Determine if  the given source-cube is suitable for concatenation
@@ -1158,7 +1164,7 @@ def register(
 
         Returns
         -------
-        bool
+        tuple[bool, str]
 
         """
         # Verify and assert the nominated axis.
@@ -1275,15 +1281,7 @@ def check_coord_match(coord_type: str) -> tuple[bool, str]:
             if existing_order == _CONSTANT and this_order != _CONSTANT:
                 self._coord_signature.dim_order[dim_ind] = this_order
 
-        if mismatch_error_msg and not match:
-            if error_on_mismatch:
-                raise iris.exceptions.ConcatenateError([mismatch_error_msg])
-            else:
-                warnings.warn(
-                    mismatch_error_msg, category=iris.warnings.IrisUserWarning
-                )
-
-        return match
+        return match, mismatch_error_msg
 
     def _add_skeleton(self, coord_signature, data):
         """Create and add the source-cube skeleton to the :class:`_ProtoCube`.

lib/iris/tests/unit/cube/test_CubeList.py

@@ -77,6 +77,21 @@ def test_empty(self):
         with pytest.raises(ValueError, match=exc_regexp):
             CubeList([]).concatenate_cube()
 
+    def test_multidim(self):
+        lat = DimCoord(np.arange(20), standard_name="latitude")
+        lon = DimCoord(np.arange(20), standard_name="longitude")
+
+        cube = Cube(np.zeros([20, 20]))
+        cube.add_dim_coord(lat, 0)
+        cube.add_dim_coord(lon, 1)
+
+        cube_quarters = CubeList(
+            [cube[:10, :10], cube[:10, 10:], cube[10:, :10], cube[10:, 10:]]
+        )
+
+        result = cube_quarters.concatenate_cube()
+        assert cube == result
+
 
 class Test_extend:
     @pytest.fixture(autouse=True)