Fix guess_bounds precision for circular coords at numpy v2

Author: rcomer

docs/src/whatsnew/latest.rst

@@ -36,8 +36,9 @@ This document explains the changes made to Iris for this release
 🐛 Bugs Fixed
 =============
 
-#. N/A
-
+#. `@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 +2402,9 @@ 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 = np.float64(self.units.modulus)
+                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):