[swift-evolution] [Draft] Abolish IUO type

[Brent Royal-Gordon]

> The most significant one is that I think that describing this in terms of an IUO attribute (which is an internal implementation detail) makes the proposal more confusing than necessary for the non-compiler hackers. :-)

For what it's worth, I actually find it really helpful to have a way to talk about these kinds of features without sugar. I spent most of this proposal going "So, does that mean `Int!` is now `@implicitly_unwrapped Optional<Int>`?"

It's particularly important in this case because `!` implies `?`; there's a lot of magic going on here. Without a completely unambiguous way to write about this behavior, it's hard to talk about it precisely.

For instance, I have two questions for the proposal's author:

* Would the declaration `Int?!` mean  `@implicitly_unwrapped Optional<Int>` or `@implicitly_unwrapped Optional<Optional<Int>>`?

* You say that things like `[Int!]` and `(Int!, Int!)` (`Array<@implicitly_unwrapped Optional<Int>>` and `(@implicitly_unwrapped Optional<Int>, @implicitly_unwrapped Optional<Int>)`) are illegal; are `Int!?` and `Int!!` (`Optional<@implicitly_unwrapped Optional<Int>>` and `@implicitly_unwrapped Optional<@implicitly_unwrapped Optional<Int>>`) illegal as well? (Actually, having written this out explicitly, I'm now fairly confident of the answer: yes, they are illegal.)

These kinds of questions are hard to pose unless we have a language to discuss them in, even if that language is rarely actually used when we write code.

-- 
Brent Royal-Gordon
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