slicing non-monotonously increasing dimension returns empty slice silently

[poplarShift]

When slicing over a dimension that is not ordered monotonously increasing, .sel returns an empty slice (which is ok) but it also doesn't complain (which is not very helpful).

import xarray as xr
print(xr.__version__)

ds = xr.tutorial.open_dataset('rasm')

print('right order selects correctly:')
print(ds.sel(x=slice(10,15)))

ds['x'] = ds.x[::-1]

print('wrong order returns empty slice, silently:')
print(ds.sel(x=slice(10,15)))

Returns:

0.12.1

right order selects correctly:
<xarray.Dataset>
Dimensions:  (time: 36, x: 5, y: 205)
Coordinates:
  * time     (time) object 1980-09-16 12:00:00 ... 1983-08-17 00:00:00
    xc       (y, x) float64 ...
    yc       (y, x) float64 ...
Dimensions without coordinates: x, y
Data variables:
    Tair     (time, y, x) float64 ...
Attributes:
    title:                     /workspace/jhamman/processed/R1002RBRxaaa01a/l...
    institution:               U.W.
    source:                    RACM R1002RBRxaaa01a
    output_frequency:          daily
    output_mode:               averaged
    convention:                CF-1.4
    references:                Based on the initial model of Liang et al., 19...
    comment:                   Output from the Variable Infiltration Capacity...
    nco_openmp_thread_number:  1
    NCO:                       "4.6.0"
    history:                   Tue Dec 27 14:15:22 2016: ncatted -a dimension...
wrong order returns empty slice, silently:
<xarray.Dataset>
Dimensions:  (time: 36, x: 0, y: 205)
Coordinates:
  * time     (time) object 1980-09-16 12:00:00 ... 1983-08-17 00:00:00
    xc       (y, x) float64 ...
    yc       (y, x) float64 ...
  * x        (x) int64 
Dimensions without coordinates: y
Data variables:
    Tair     (time, y, x) float64 ...
Attributes:
    title:                     /workspace/jhamman/processed/R1002RBRxaaa01a/l...
    institution:               U.W.
    source:                    RACM R1002RBRxaaa01a
    output_frequency:          daily
    output_mode:               averaged
    convention:                CF-1.4
    references:                Based on the initial model of Liang et al., 19...
    comment:                   Output from the Variable Infiltration Capacity...
    nco_openmp_thread_number:  1
    NCO:                       "4.6.0"
    history:                   Tue Dec 27 14:15:22 2016: ncatted -a dimension...

[dcherian]

Thanks for opening an issue @poplarShift.

There's been quite some discussion in #1613, so I'll close this in favour of that. Please chime in there.

[poplarShift]

Thanks! Thought I looked everywhere if the issue already existed but must have missed that.