swift build --triple does not affect build target

[krzyzanowskim]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

Triple specified with the "triple" argument is not picked up by build:

swift build --triple arm64-apple-macosx15.0
Target: arm64-apple-macosx10.13

and specify target triple for swiftc is picked up by build

swift build -Xswiftc -target -Xswiftc arm64-apple-macosx15.0
Target: arm64-apple-macosx15.0

Expected behavior

swift build --triple triple-value override/set Package manifest platform target triple

Actual behavior

--triple value is ignored

Steps to reproduce

on macOS, build simple package with minimum manifest

// swift-tools-version:5.9
import PackageDescription

let package = Package(
    name: "Foo",
    products: [
        .executable(
            name: "Foo",
            targets: ["Foo"]),
    ],
    dependencies: [],
    targets: [
        .executableTarget(
            name: "Foo",
            dependencies: []),
    ]
)

swift build --triple arm64-apple-macosx15.0 -v
Target: arm64-apple-macosx10.13

Swift Package Manager version/commit hash

Swift Package Manager - Swift 6.1.0

Swift & OS version (output of swift --version ; uname -a)

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0
Darwin MarcinK-MacBook.local 24.5.0 Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:33 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T8122 arm64

[dschaefer2]

What if you set the platform version for macOS to .v15 in your Package.swift? 10.13 is the current default.

[krzyzanowskim]

the Package.swift manifest takes precedence over the swift build --triple but not over the swift build -Xswiftc -target -Xswiftc arm64-apple-macosx15.0

[rauhul]

Related (IMO) swift build should always build for the host triple and the version in platforms just indicates the minimum that could be used. choosing to build for an older version should be an explicit choice rather than an artifact of supporting old versions.

[finagolfin]

I was confused by this report, as this flag had worked for me before, so I looked into it. Turns out this is a very limited flag that was only added as a CLI override for when a --destination flag is specified, in #2604 five years ago. Apparently that's why those flags have never been documented to this day.

I believe I tried them out in the intended context back then and they do work for that. All notions of their being useful in other contexts, eg with SDK bundles, are incorrect: that has never been implemented, see #7330.