[swift-evolution] [Pitch] Make the formal type of 'self' consistent in class methods

[Slava Pestov]

Consistent formal type for 'self' in class methods

Proposal: SE-9999 <https://github.com/slavapestov/swift-evolution/blob/self-formal-type-in-class/proposals/9999-self-formal-type-in-class.md>
Author: Slava Pestov <https://github.com/slavapestov>
Status: Awaiting review
Review manager: TBD

 <https://github.com/slavapestov/swift-evolution/tree/self-formal-type-in-class#introduction>Introduction

This proposal makes the self value behave consistently whether or not it is used from a method with a Self return type.

Swift-evolution thread: Discussion thread topic for that proposal <http://news.gmane.org/gmane.comp.lang.swift.evolution>
 <https://github.com/slavapestov/swift-evolution/tree/self-formal-type-in-class#motivation>Motivation

Right now, we exhibit inconsistent behavior when self is used as an argument to a generic function, violating the principle of least surprise.

Consider the following code:

class Base {
  @discardableResult
  func methodWithDynamicSelf() -> Self {
    doSomething(self)
    return self
  }

  func methodWithoutDynamicSelf() {
    doSomething(self)
  }
}

class Derived : Base {}

func doSomething<T>(_ t: T) {
  print(T.self)
}

Base().methodWithDynamicSelf()
Base().methodWithoutDynamicSelf()

Derived().methodWithDynamicSelf()
Derived().methodWithoutDynamicSelf()
Currently, it prints the following output:

Base
Base
Derived
Base
Note that there's no inconsistency when the method is called on the base class. When called on the derived class however, we see that in a method with a dynamic Self return type, the type of self is Derived, whereas in a method with any other return type, the type of self is Base.


 <https://github.com/slavapestov/swift-evolution/tree/self-formal-type-in-class#proposed-solution>Proposed solution

The proposal is to change the type of self to always be Self, which can be thought of as a special generic type parameter bound to the dynamic type of the instance.

With this proposal, the above code will instead produce the following:

Base
Base
Derived
Derived
Here, the type of self would always be Derived when called on an instance of the derived class.

Of course a more useful program could instead do something with the type parameter T, such as constraining it to a protocol or a class with a required initializer, and then using the type to construct a new instance of the class.

This also dovetails nicely with SE-0068 <https://github.com/slavapestov/swift-evolution/blob/self-formal-type-in-class/proposals/0068-universal-self.md>.

Finally, it opens the door to generalizing dynamic Self, allowing it to appear in covariant position within parameter types:

class ArtClass {
  func paint(withBrush: (Self) -> ()) { ... }
}
This would allow a class to conform to a protocol with a requirement written like the following, something that is currently not possible at all:

protocol OddProtocol {
  func weaken<X, Y>((Self) -> (X) -> Y) -> (X) -> Y
}

 <https://github.com/slavapestov/swift-evolution/tree/self-formal-type-in-class#detailed-design>Detailed design

There's really not much more to say here. The code for typing self with a dynamic Self is in place already, however enabling this change might expose some new bugs we have not yet encountered, because currently, methods with dynamic Self return type are relatively rare.


 <https://github.com/slavapestov/swift-evolution/tree/self-formal-type-in-class#impact-on-existing-code>Impact on existing code

This will have a small impact on existing code that uses a pattern similar to the above.


 <https://github.com/slavapestov/swift-evolution/tree/self-formal-type-in-class#alternatives-considered>Alternatives considered

One alternative is to simply do nothing, but this makes the language less consistent than it could be.
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