…n field specifiers (#15010)(Basic functionality was implemented by@wesleywright in#14870. I addedoverload resolution.)This note from PEP 681 was missed in the initial implementation of field specifiers:> If unspecified, init defaults to True. Field specifier functions canuse overloads that implicitly specify the value of init using a literalbool value type (Literal[False] or Literal[True]).This commit adds support for reading a default from the declared type ofthe `init` parameter if possible. Otherwise, it continues to use thetypical default of `True`.The implementation was non-trivial, since regular overload resolutioncan't be used in the dataclass plugin, which is applied before typechecking. As a workaround, I added a simple overload resolution helperthat should be enough to support typical use cases. It doesn't do fulloverload resolution using types, but it knows about `None`,`Literal[True]` and `Literal[False]` and a few other things.---------Co-authored-by: Wesley Collin Wright <wesleyw@dropbox.com>