Check for just ..., rather than [...] in da.stack (#6132)

* Add tests for cases discussed in issue 6051

* Raise error if dims = ...

xarray/core/dataset.py

@@ -3870,6 +3870,8 @@ def reorder_levels(
         return self._replace(variables, indexes=indexes)
 
     def _stack_once(self, dims, new_dim):
+        if dims == ...:
+            raise ValueError("Please use [...] for dims, rather than just ...")
         if ... in dims:
             dims = list(infix_dims(dims, self.dims))
         variables = {}

xarray/tests/test_dataarray.py

@@ -6678,3 +6678,17 @@ def test_from_pint_wrapping_dask(self):
         expected = xr.DataArray(arr, dims="x", coords={"lat": ("x", arr * 2)})
         assert_identical(result, expected)
         np.testing.assert_equal(da.to_numpy(), arr)
+
+
+class TestStackEllipsis:
+    # https://github.com/pydata/xarray/issues/6051
+    def test_result_as_expected(self):
+        da = DataArray([[1, 2], [1, 2]], dims=("x", "y"))
+        result = da.stack(flat=[...])
+        expected = da.stack(flat=da.dims)
+        assert_identical(result, expected)
+
+    def test_error_on_ellipsis_without_list(self):
+        da = DataArray([[1, 2], [1, 2]], dims=("x", "y"))
+        with pytest.raises(ValueError):
+            da.stack(flat=...)