问题
Migrating to Swift 3 from 2.3 and am running into this issue. The error is traceable to a view controller.
I do not see any extensions/protocols which would require a 'Key' constrained to 'String'.
I've tried to comment out code that might be causing the error, and have had strange results - ie removing an empty viewDidLoad() made the error appear in another class.
I'll update the thread if I make progress.
回答1:
Key is a type inside a structure maybe a struct/class like Dictionary.
Use AnyHashable as Key to replace String if in Dictionary.
来源：https://stackoverflow.com/questions/42519477/unknown0-error-type-key-constrained-to-non-protocol-type-string