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プログラムは思った通りには動かない、
書いた通りに動く
作者不詳
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const a = '1';
const b = 2;
const c = a + b;
console.log(c); // 3?
❌
🤔
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const a = '1';
const b = 2;
const c = a + b;
console.log(c); // '12'
🤮
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const a = 1;
const b = 2;
const c = a + b;
console.log(c); // 3
😇
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import Foundation
import SamplePackage
@main
struct Runner {
static func main() {
SamplePackage.run {
print("Hello, world!")
}
let result = SamplePackage.greeting {
"c"
}
print(result == "C: c")
}
}
import Testing
@testable import SamplePackage
@Test func run() async throws {
SamplePackage.run {
print("Hello, world!")
}
}
@Test func greeting() async throws {
let result = SamplePackage.greeting {
"c"
}
#expect(result == "C: c")
}
※サンプルコードです。実際のコードとは関係ありません。
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import Foundation
import SamplePackage
@main
struct Runner {
static func main() {
SamplePackage.run {
print("Hello, world!")
}
let result = SamplePackage.greeting {
"c"
}
print(result == "C: c")
}
}
import Testing
@testable import SamplePackage
@Test func run() async throws {
SamplePackage.run {
print("Hello, world!")
}
}
@Test func greeting() async throws {
let result = SamplePackage.greeting {
"c"
}
#expect(result == "C: c")
}
※サンプルコードです。実際のコードとは関係ありません。
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書いた通りに動いていない…
だと…？
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OSのバージョン差異
ライブラリ
Xcode/Swift
のバージョン差異
のバージョン差異
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＿人人人人人人人人＿
＞
全て同じ条件
＜
￣Y^Y^Y^Y^YY^Y^￣
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🤔
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// swift-tools-version: 6.3
import PackageDescription
let package = Package(
name: "iOSDC2026_SampleRunner",
products: [],
dependencies: [
.package(name: "iOSDC2026_Sample", path: "../"),
],
targets: [
.executableTarget(
name: "SampleRunner",
dependencies: [
.product(
name: "SamplePackage",
package: "iOSDC2026_Sample"
),
],
swiftSettings: [
.enableUpcomingFeature("ForwardTrailingClosures"),
],
),
],
swiftLanguageModes: [.v5]
)
// swift-tools-version: 6.3
import PackageDescription
let package = Package(
name: "iOSDC2026_Sample",
products: [
.library(
name: "SamplePackage",
targets: ["SamplePackage"]
),
],
targets: [
.target(
name: "SamplePackage",
),
.testTarget(
name: "SamplePackageTests",
dependencies: ["SamplePackage"]
),
],
swiftLanguageModes: [.v5]
)
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お分かりいただけただろうか
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// swift-tools-version: 6.3
import PackageDescription
let package = Package(
name: "iOSDC2026_SampleRunner",
products: [],
dependencies: [
.package(name: "iOSDC2026_Sample", path: "../"),
],
targets: [
.executableTarget(
name: "SampleRunner",
dependencies: [
.product(
name: "SamplePackage",
package: "iOSDC2026_Sample"
),
],
swiftSettings: [
.enableUpcomingFeature("ForwardTrailingClosures"),
],
),
],
swiftLanguageModes: [.v5]
)
// swift-tools-version: 6.3
import PackageDescription
let package = Package(
name: "iOSDC2026_Sample",
products: [
.library(
name: "SamplePackage",
targets: ["SamplePackage"]
),
],
targets: [
.target(
name: "SamplePackage",
),
.testTarget(
name: "SamplePackageTests",
dependencies: ["SamplePackage"]
),
],
swiftLanguageModes: [.v5]
)
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フラグが…ない…
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完
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解説
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解説
ForwardTrailingClosures(SE-0286、Swift 5.8+)
public func run(
_ a: (() -> Void)? = nil,
_ b: (() -> Void)? = nil,
_ c: (() -> Void)? = nil,
) {
if let a {
print("\u{001B}[31mRunning A\u{001B}[0m")
a()
}
if let b {
print("\u{001B}[32mRunning B\u{001B}[0m")
b()
}
if let c {
print("\u{001B}[33mRunning C\u{001B}[0m")
c()
}
}
run {
print("Hello, world!")
}
ForwardTrailingClosures = NO
ForwardTrailingClosures = YES
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解説
Upcoming Feature Flagsはコンパイルに渡すフラグ
• 各モジュールはUpcoming Feature Flagsの情報を持たない
• モジュール間で分割可能
ForwardTrailingClosures
YES
ForwardTrailingClosures
NO
A
B
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対策(同じ挙動にしたい場合)
全モジュールに適用する-Package内の全targetに適用
let package = Package(
name: "SamplePackage",
products: [],
dependencies: [~],
targets: [
.target(
name: "FeatureA",
dependencies: [~],
swiftSettings: [
.enableUpcomingFeature("ForwardTrailingClosures"),
],
),
.target(
name: "FeatureB",
dependencies: [~],
swiftSettings: [
.enableUpcomingFeature("ForwardTrailingClosures"),
],
),
.target(
name: "FeatureC",
dependencies: [~],
swiftSettings: [
.enableUpcomingFeature("ForwardTrailingClosures"),
],
),
…
],
)
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対策(同じ挙動にしたい場合)
全モジュールに適用する-Package内の全targetに適用
let package = Package(
name: "SamplePackage",
products: [],
dependencies: [~],
targets: [
.target(
name: "FeatureA",
dependencies: [~],
),
.target(
name: "FeatureB",
dependencies: [~],
),
.target(
name: "FeatureC",
dependencies: [~],
),
…
],
)
package.targets.forEach {
$0.swiftSettings = [
.enableUpcomingFeature("ForwardTrailingClosures"),
]
}
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対策(同じ挙動にしたい場合)
全モジュールに適用する-複数Packageを跨ぐ場合
• 仕組みで検知するのは難しい🥺
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そこでAIの出番🤖
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まとめ
• 書いたとおりに動いていないと思っていたけど結局書いた通りに動いていた
• Package.swiftやxcodeprojのBuild Settingsも疑おう
• そもそもそんな変な実装するな
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もう対応終わってるし！
なんならSwift 6対応できてるし！
😤
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NonisolatedNonsendingByDefault
ImmutableWeakCaptures
MemberImportVisibility
Swift 7対応が待ってます😇
InferIsolatedConformances
InternalImportsByDefault
ExistentialAny
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Self Introduction
• 417.72KI
• Twitter: @417̲72ki
• GitHub: 417-72KI
• Icon by: @fredii
• Working at: (REDACTED)
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おまけ
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コンパイル時に評価される過程
SILで差分確認
$ swiftc -emit-silgen -swift-version 5 sample.swift > sample-5.sil
$ swiftc -emit-silgen -swift-version 6 sample.swift > sample-6.sil
import Foundation
public func run(
_ a: (() -> Void)? = nil,
_ b: (() -> Void)? = nil,
_ c: (() -> Void)? = nil,
) {}
run { print("Hello world!") }
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プログラムは思った通りには動かない。
書いた通りに動k...
動かない...だと...？
2026/09/13 iOSDC 2026 Day 2