ファイル単位のデフォルト

// LegacyLogger.swift

default @available(*, deprecated, message: "this logging system uses global state")

// Implicitly @available(*, deprecated, message: "this logging system uses global state")
func log(message: String) { ... }

// Implicitly @available(*, deprecated, message: "this logging system uses global state")
func initLogging() { ... }

func f() {
  default @MainActor // error
}

default @available(swift, introduced: 5.9) // legal

import MyLibrary

default @diagnose(StrictMemorySafety, as: error) // legal

func foo() {}

default nonisolated // error

default @MainActor
default @MainActor // error

default nonisolated
default @available(swift, introduced: 5.1)
default @MainActor // error

default @MainActor

// Implicitly '@MainActor'
struct S {}

// Implicitly '@MainActor'
extension S {
  // Implicitly the same as the extension, which is implicitly '@MainActor'
  func foo() {}
}

// still 'nonisolated'
nonisolated extension S {
  // Implicitly the same as the extension, which is explicitly 'nonisolated'
  func bar() {}
}

// still 'nonisolated'
nonisolated struct T {}

default @MainActor

// Implicitly '@MainActor'
class S { ... }

// Implicitly '@MainActor'
extension S: Hashable { ... } // error: Conformance of 'S' to protocol 'Hashable' crosses into main actor-isolated code and can cause data races

// In MyLibrary

@MainActor
class S { ... }

// In MyClient compiled with -default-isolation MainActor
import MyLibrary

default nonisolated

// Implicitly nonisolated, even though S is isolated to @MainActor
extension S {
  // Accessing @MainActor isolated state from S would be an error
}

// script mode

default nonisolated

// implicitly nonisolated
var count = 0 // error, mutable global is neither isolated nor immutable+Sendable

// implicitly nonisolated
let limit = 100 // allowed

// Legacy.swift

default @diagnose(DeprecatedDeclaration, as: ignored, reason: "Maintaining backwards compatibility")

// Implicitly '@diagnose(DeprecatedDeclaration, as: ignored)'
func restoreLegacyAPI() {
  deprecatedHelper()
}

// Implicitly '@diagnose(DeprecatedDeclaration, as: ignored)'
extension Logger {
  func legacyFormat(_ s: String) -> String {
    oldStringFormatter(s)
  }
}

// explicit '@diagnose' takes precedence over the default
@diagnose(DeprecatedDeclaration, as: warning)
func migrationBridge() {
  anotherDeprecatedHelper()
}

default @diagnose(DiagGroupID, as: warning)
default @diagnose(DiagGroupID, as: error)

// implicit @diagnose(DiagGroupID, as: warning)
// implicit @diagnose(DiagGroupID, as: error)
public func foo() // DiagGroupID is diagnosed as an error

// implicit @diagnose(DiagGroupID, as: warning)
// implicit @diagnose(DiagGroupID, as: error)
@diagnose(DiagGroupID, as: ignored)
public func bar() // DiagGroupID is ignored

// LockedCollections.swift

default @available(*, noasync, message: "holding a lock across suspension may deadlock")

// Implicitly '@available(*, noasync, message: "holding a lock across suspension may deadlock")'
public final class LockedEventLog {
  public func lock() { ... }
  public func unlock() { ... }
  public func append(_ event: String) { ... }
  public func entries() -> [String] { ... }
}

// Implicitly '@available(*, noasync, message: "holding a lock across suspension may deadlock")'
public final class LockedCache<Key: Hashable, Value> {
  public func lock() { ... }
  public func unlock() { ... }
  public func get(_ key: Key) -> Value? { ... }
  public func set(_ key: Key, to value: Value) { ... }
}

default @available(macOS 12, *)

// Has default macOS 12 availability
public protocol Widget { ... }

// Has default macOS 12 availability
public struct BasicWidget: Widget { ... }

// Has explicit macOS 14 availability
@available(macOS 14, *)
public protocol AdvancedWidget { ... }

// Inherits macOS 14 availability from the extended protocol
extension AdvancedWidget { ... }