id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
957918751,MDU6SXNzdWU5NTc5MTg3NTE=,5664,Interpolation behaviour inconsistent with numpy? ,7017525,open,0,,,4,2021-08-02T08:56:28Z,2024-03-12T01:15:46Z,,NONE,,,,"Hey all, 
When running `dataset.interp(time=dataset.time)` fills with `np.nan` if one of the neighbor is a `np.nan` **even when interpolation is not actually needed**.

Here is the sample code to reproduce the issue : 
```python 
def test_crop_times_nan() :
        ds = xr.Dataset(
            data_vars = {
                ""some_variable"" : (['x', 'time'], np.array([[np.nan, 0, 1]]))
            },
            coords = {
                ""time"" : np.array([0,1,2])
            }
        )
        result = ds.interp(time=ds.time)

        # result[""some_variable""].value == [nan, nan, 1.0]
        # whereas [nan, 0, 1.0] is EXPECTED
        xr.testing.assert_allclose(ds, result)
```
Please note that numpy does not have the same behavior :  
```python
>>> import numpy as np
>>> np.interp([0,1,2], xp=[0,1,2], fp=[np.nan,0,1])
array([nan,  0.,  1.])
```

Is that an intended behaviour for xarray? 
If so, does this mean that I first have to check if an interpolation is needed instead of doing it no matter what (and use `reindex` instead of `interp` if it is not needed) ?  
(this will be kind of tricky if interpolation is needed for certain values and some not...)

Thanks for your help ;) 

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>
INSTALLED VERSIONS
------------------
commit: None
python: 3.8.5 (default, Jul 28 2020, 12:59:40) 
[GCC 9.3.0]
python-bits: 64
OS: Linux
OS-release: 5.8.0-7642-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.0
libnetcdf: 4.7.4

xarray: 0.18.2
pandas: 1.2.4
numpy: 1.19.4
scipy: 1.6.0
netCDF4: 1.5.6
pydap: None
h5netcdf: 0.8.1
h5py: 3.1.0
Nio: None
zarr: None
cftime: 1.3.0
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2021.01.0
distributed: 2021.01.0
matplotlib: 3.4.2
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 57.4.0
pip: 20.2.4
conda: None
pytest: None
IPython: 7.19.0
sphinx: None


</details>
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