Some interesting things brought up and I agree with a lot, disagree with a few 
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My biggest wish to fix if we are talking about breaking changes is Default Protocol Implementation Inheritance Behaviour - The current situation and what/if anything should be done about it Its tripped a lot of people up and addressing it would smooth out a rough and very unexpected area of swift.
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I might be one of the few people who actually like the basic
Codable, but as soon as you move outside of a 'standard' workflow it fails quickly. It would be great to address some areas around this to make it much more flexible and hopefully gain some speed. Some lessons could be learnt from Rust's Serde. -
I also really miss having named arguments in tuples. I understand the arguments as to why they were removed, but it just feels like removing useful information from the language.