Summary: The descriptor `__set_name__` functionality (introduced in Python 3.6) does not seem to be working correctly for `dataclass.Field` objects with a default pointing to a descriptor. I have attached a file demonstrating the trouble.

Details: If I set a `dataclass` class object field to a `dataclass.field` with a descriptor object for the `default` argument, the descriptor's `__set_name__` method is not called during initialization. This is unexpected because descriptors themselves seem to work pretty much flawlessly, otherwise. 

(Bravo on that by the way! Working descriptors isn't mentioned at all in the PEP as a feature but I was very pleased to see them working!!)

System details:
Python 3.7b02
Windows 10
PyCharm Community Edition

btw this is my first ever Python bug report; I hope I did a good job.