[swift-evolution] [Pitch] Nil struct

[Anton Zhilin]

Gist link <https://gist.github.com/Anton3/ba56a29986c59e9595368be3cb02fb1b>
Introduction

Change nil literal type from () to Nil.
Before:

public protocol ExpressibleByNilLiteral {
  init(nilLiteral: ())
}

After:

public struct Nil {
  init()
}
public protocol ExpressibleByNilLiteral {
  associatedtype NilLiteralType = Nil
  init(nilLiteral: NilLiteralType)
}

Motivation

Currently, nil differs from other literals: it doesn’t have its own type.
But in some cases we want to deal directly with it, without creating any
instances.

The most important use case is comparison of an Optional to nil.
Currently, this operation is implemented using a hidden struct
_OptionalNilComparisonType,
which is needed precisely because because nil does not have its own type.
Removal of such underscored types is one of the goals stated in Generics
manifesto.

Additionally, declaration of ExpressibleByNilLiteral differs from all other
Expressibles,
because it doesn’t have a Literal type. It is generally beneficial to
eliminate special cases.
Proposed solution

Introduce a struct Nil, which becomes the default type for nil literals:

public struct Nil : ExpressibleByNilLiteral {
  init()
  init(nilLiteral: NilLiteralType)
}
let a = nilprint(type(of: a))   //=> Nil

Rewrite ExpressibleByNilLiteral:

public protocol ExpressibleByNilLiteral {
  associatedtype NilLiteralType = Nil
  init(nilLiteral: NilLiteralType)
}

Make use of Nil in the standard library:

public func == <T>(left: T?, right: Nil)public func == <T>(left: Nil,
right: T?)public func != <T>(left: T?, right: Nil)public func !=
<T>(left: Nil, right: T?)public func ~= <T>(left: Nil, right: T?)

Source compatibility

Nil identifier is taken, therefore applications that already use it will
stop compiling.
Automatic migration is somewhat possible by renaming of the old entity;
manual migration is recommended.

Applications that use declare ExpressibleByNilLiteral conformances will
stop compiling.
Automatic migration is possible.
Effect on ABI stability

Applications that use Nil identifier will have to make ABI-breaking changes.

Otherwise, the change can mostly be applied in an ABI-compatible manner.
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