Fix resampling codes.

xarray/core/groupby.py

@@ -292,7 +292,12 @@ def _get_index_and_items(index, grouper):
         # This way we generate codes for the final output index: full_index.
         # So for _flox_reduce we avoid one reindex and copy by avoiding
         # _maybe_restore_empty_groups
-        codes = first_items.index.searchsorted(index, side=grouper.closed)
+        codes = (
+            first_items.index.searchsorted(
+                index, side="right" if grouper.closed == "left" else "left"
+            )
+            - 1
+        )
         _apply_loffset(grouper, first_items)
     full_index = first_items.index
     if first_items.isnull().any():

xarray/core/resample_cftime.py

@@ -91,7 +91,12 @@ def first_items(self, index):
         datetime_bins, labels = _get_time_bins(
             index, self.freq, self.closed, self.label, self.base
         )
-        codes = np.searchsorted(labels, index, side=self.closed)
+        codes = (
+            np.searchsorted(
+                labels, index, side="right" if self.closed == "left" else "left"
+            )
+            - 1
+        )
         if self.loffset is not None:
             if isinstance(self.loffset, datetime.timedelta):
                 labels = labels + self.loffset

xarray/tests/test_groupby.py

@@ -1480,6 +1480,10 @@ def test_resample(self):
         actual = array.resample(time="24H").reduce(np.mean)
         assert_identical(expected, actual)
 
+        actual = array.resample(time="24H", closed="right").mean()
+        expected = DataArray(array.to_series().resample("24H", closed="right").mean())
+        assert_identical(expected, actual)
+
         # Our use of `loffset` may change if we align our API with pandas' changes.
         # ref https://github.com/pydata/xarray/pull/4537
         actual = array.resample(time="24H", loffset="-12H").mean()