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
617140674,MDU6SXNzdWU2MTcxNDA2NzQ=,4054,Type checking fails for multiplication,37403847,closed,0,,,6,2020-05-13T04:55:14Z,2021-04-14T15:59:59Z,2021-04-14T15:59:59Z,CONTRIBUTOR,,,,"<!-- A short summary of the issue, if appropriate -->


#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

I save the following in a file called ""foo.py""
```python
import xarray as xr

def mul(x: float, y: xr.Dataset):
    return x*y

```
and run
```
mypy foo.py
```

#### Expected Output

This should give
```
Success: no issues found in 1 source file
```

#### Problem Description
<!-- this should explain why the current behavior is a problem and why the expected output is a better solution -->

Instead I get the output
```
foo.py:4: error: Unsupported operand types for * (""float"" and ""Dataset"")
```
It gives a similar error for addition.
If I reverse the order of multiplication it works as expected, namely
```python
import xarray as xr

def mul(x: float, y: xr.Dataset):
    return y*x

```
gives
```
Success: no issues found in 1 source file
```

But it would be nice if it also worked when the float is first, so I don't have to change a lot existing code to get typechecking to work.

#### Versions
I tested on version 0.15.1 and on the current master

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

<!-- Paste the output here xr.show_versions() here -->
INSTALLED VERSIONS
------------------
commit: None
python: 3.8.2 (default, May  7 2020, 20:00:49) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 5.4.0-29-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.15.2.dev61+gbd84186a
pandas: 1.0.3
numpy: 1.18.4
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 46.2.0.post20200511
pip: 20.0.2
conda: None
pytest: None
IPython: None
sphinx: None


</details>
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