Fix guess_bounds precision for circular coords at numpy v2 (#6793)

* Fix guess_bounds precision for circular coords at numpy v2

* Apply review suggestion

Co-authored-by: Martin Yeo <[email protected]>

---------

Co-authored-by: Martin Yeo <[email protected]>

Author: rcomer

docs/src/whatsnew/3.14.rst

@@ -46,6 +46,19 @@ This document explains the changes made to Iris for this release
    And finally, get in touch with us on :issue:`GitHub<new/choose>` if you have
    any issues or feature requests for improving Iris. Enjoy!
 
+v3.14.1 (Date TBD)
+==================
+
+.. dropdown:: v3.14.1 Patches
+   :color: primary
+   :icon: alert
+   :animate: fade-in
+   :open:
+
+   The patches in this release of Iris include:
+
+   #. Fixed compatibility with NumPy v2 for :meth:`~iris.coords.Coord.guess_bounds`
+      :ref:`See the full entry for more<3_14_1_guess_bounds>`.
 
 ✨ Features
 ===========
@@ -112,6 +125,12 @@ This document explains the changes made to Iris for this release
    error, caused by bad synchronisation between Python-layer and HDF-layer
    file locking on certain filesystems. (:pull:`6760`).
 
+.. _3_14_1_guess_bounds:
+
+#. `@rcomer`_ ensured that :meth:`~iris.coords.Coord.guess_bounds` calculates at
+   double precision for circular coordinates so the bounds span the full circle
+   (:issue:`6738`, :pull:`6793`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/coords.py

@@ -2402,8 +2402,12 @@ def _guess_bounds(self, bound_position=0.5, monthly=False, yearly=False):
                 points = np.empty(self.shape[0] + 2)
                 points[1:-1] = self.points
                 direction = 1 if self.points[-1] > self.points[0] else -1
-                points[0] = self.points[-1] - (self.units.modulus * direction)
-                points[-1] = self.points[0] + (self.units.modulus * direction)
+                modulus_type = np.promote_types(
+                    self.points.dtype, type(self.units.modulus)
+                )
+                (modulus,) = np.array([self.units.modulus], dtype=modulus_type)
+                points[0] = self.points[-1] - (modulus * direction)
+                points[-1] = self.points[0] + (modulus * direction)
                 diffs = np.diff(points)
             else:
                 diffs = np.diff(self.points)

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

@@ -194,6 +194,15 @@ def test_circular_decreasing_alt_range(self):
         )
         _shared_utils.assert_array_equal(target, self.coord.bounds)
 
+    def test_circular_n216_float32(self):
+        # Values represent longitude taken from an N216 pp-file.
+        coord = DimCoord.from_regular(
+            -0.41666666, 0.8333333, 432, units="degrees", circular=True
+        )
+        coord.guess_bounds()
+        # The last bound should match up to the first.
+        assert coord.bounds[-1, -1] - coord.bounds[0, 0] == 360
+
 
 class Test_guess_bounds__default_enabled_latitude_clipping:
     def test_all_inside(self):