Description

The decorator quantity_input() tries to convert the annotated return value to a Quantity here. When a return annotation other than Quantity is used, this conversion fails of course resulting in the error below.

Expected behavior

The decorator quantity_input() should ignore annotated return values which are not of type Quantity. This has also been disicussed and fixed here but the fix has been discarded.

Actual behavior

The following error message is shown:

Traceback (most recent call last):
  File "<input>", line 6, in <module>
  File "<<redacted>>/venv/lib/python3.7/site-packages/astropy/units/decorators.py", line 236, in wrapper
    return return_.to(wrapped_signature.return_annotation)
AttributeError: 'str' object has no attribute 'to'

Steps to Reproduce

1. [Run the code below in a python console]

import astropy.units as u

@u.quantity_input(temp=[u.Kelvin, u.Celsius])
def foo(temp: u.Quantity) -> str:
    return "bar"
print(foo(300 * u.K)())

System Details

Darwin-19.4.0-x86_64-i386-64bit
Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 16:52:21) 
[Clang 6.0 (clang-600.0.57)]
Numpy 1.18.2
astropy 4.0.1.post1
Scipy 1.4.1