Preserve masked bounds in Coord.cell (#5158)

docs/src/whatsnew/latest.rst

@@ -36,7 +36,10 @@ This document explains the changes made to Iris for this release
 🐛 Bugs Fixed
 =============
 
-#. N/A
+#. :user:`gaoflow` fixed :meth:`iris.coords.Coord.cell` so that a masked bound
+   is preserved as masked, instead of revealing the value stored underneath the
+   mask. This also corrects the cube/coordinate printout for such bounds.
+   (:issue:`5158`)
 
 
 💣 Incompatible Changes

lib/iris/coords.py

@@ -2235,7 +2235,10 @@ def cell(self, index):
 
         bound = None
         if self.has_bounds():
-            bound = tuple(np.array(self.core_bounds()[index], ndmin=1).flatten())
+            # Use `np.asanyarray` to preserve any masked values (see #5158):
+            bound = tuple(
+                np.atleast_1d(np.asanyarray(self.core_bounds()[index])).flatten()
+            )
 
         if self.units.is_time_reference():
             point = self.units.num2date(point)

lib/iris/tests/unit/coords/test_Coord.py

@@ -445,6 +445,22 @@ def test_time_as_object(self, mocker):
             mocker.call((mocker.sentinel.lower, mocker.sentinel.upper)),
         ]
 
+    def test_masked_point(self):
+        # A masked point should be preserved as masked in the cell (#5158).
+        coord = AuxCoord(ma.masked_array([3.0], mask=[True]))
+        cell = coord.cell(0)
+        assert cell.point is ma.masked
+
+    def test_masked_bound(self):
+        # A masked bound should be preserved as masked in the cell, rather
+        # than revealing the value underneath the mask (#5158).
+        coord = AuxCoord(
+            [5.0], bounds=ma.masked_array([[3.0, 7.0]], mask=[[True, False]])
+        )
+        cell = coord.cell(0)
+        assert cell.bound[0] is ma.masked
+        assert cell.bound[1] == 7.0
+
 
 class Test_collapsed(CoordTestMixin):
     def test_serialize(self):