[swift-evolution] [Pitch] Simpler interpretation of a reference to a generic type with no arguments

[Slava Pestov]

Simpler interpretation of a reference to a generic type with no arguments

Proposal: SE-9999 <https://github.com/slavapestov/swift-evolution/blob/silly-proposals/proposals/9999-simplify-unbound-generic-type.md>
Author: Slava Pestov <https://github.com/slavapestov>
Status: Awaiting review
Review manager: TBD
 <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#introduction>Introduction

This proposal cleans up the semantics of a reference to a generic type when no generic arguments are applied.

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/silly-proposals/proposals#motivation>Motivation

Right now, we allow a generic type to be referenced with no generic arguments applied in a handful of special cases. The two primary rules here are the following:

If the scope from which the reference is made is nested inside the definition of the type or an extension thereof, omitting generic arguments just means to implicitly apply the arguments from context.

For example,

struct GenericBox<Contents> {
  let contents: Contents

  // Equivalent to: func clone() -> GenericBox<Contents>
  func clone() -> GenericBox {
    return GenericBox(contents: contents)
  }
}

extension GenericBox {
  func print() {
    // Equivalent to: let cloned: GenericBox<Contents>
    let cloned: GenericBox = clone()
    print(cloned.contents)
  }
}
If the type is referenced from an unrelated scope, we attempt to infer the generic parameters.

For example,

func makeABox() -> GenericBox<Int> {
  // Equivalent to: GenericBox<Int>(contents: 123)
  return GenericBox(contents: 123)
}
The problem appears when the user expects the second behavior, but instead encounters the first. For example, the following does not type check:

extension GenericBox {

  func transform<T>(f: Contents -> T) -> GenericBox<T> {
    // We resolve 'GenericBox' as 'GenericBox<Contents>', rather than
    // inferring the type parameter
    return GenericBox(contents: f(contents))
  }
}
 <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#proposed-solution>Proposed solution

The proposed solution is to remove the first rule altogether. If the generic parameters cannot be inferred from context, they must be specified explicitly with the usual Type<Args...> syntax.

 <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#detailed-design>Detailed design

This really just involves removing an existing piece of logic from the type resolver code.

 <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#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/silly-proposals/proposals#alternatives-considered>Alternatives considered

 <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#status-quo>Status quo

We could keep the current behavior, but one can argue it is not very useful, and adds a special case where one is not needed.

 <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#more-complex-inference-of-generic-parameters>More complex inference of generic parameters

We could attempt to unify the two rules for resolving a reference to a generic type with no arguments, however this presents theoretical difficulties with our constraint solver design. Even if it were easy to implement, it would increase type checking type by creating new possibilities to consider, with very little actual benefit.
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