SwiftPM selects wrong architecture from multi-arch Swift SDK when using --swift-sdk and --triple

[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

When building with a multi-architecture Swift SDK using both --swift-sdk and --triple, SwiftPM nondeterministically selects the wrong architecture variant from the SDK bundle, ignoring the target triple specified via --triple.

The SDK bundle contains both x86_64 and aarch64 variants, and even though --triple aarch64-swift-linux-musl is specified, SwiftPM sometimes selects the x86_64 variant instead, causing build failures.

Expected behavior

SwiftPM should select the SDK variant matching the --triple argument (aarch64) from the multi-arch SDK bundle.

Actual behavior

SwiftPM nondeterministically selects between x86_64 and aarch64 variants

Steps to reproduce

1. Install a multi-architecture Swift SDK (SDK contains both x86_64-swift-linux-musl and aarch64-swift-linux-musl)
   $ swift sdk list
   # Output: swift-6.2-RELEASE_static-linux-0.0.1
2. Create a minimal test package:
   $ mkdir -p /tmp/swiftpm-bug-repro && cd /tmp/swiftpm-bug-repro
   $ swift package init --type executable --name TestApp
3. Build for aarch64 Linux:
   $ swift build --swift-sdk swift-6.2-RELEASE_static-linux-0.0.1 --triple aarch64-swift-linux-musl -Xswiftc -static-executable
4. Observe the warning and error:
   warning: multiple Swift SDKs match ID swift-6.2-RELEASE_static-linux-0.0.1 and host triple arm64-apple-macosx26.0
   error: could not find module '_Concurrency' for target 'aarch64-swift-linux-musl';
   found: x86_64-swift-linux-musl, at: .../x86_64/usr/lib/swift_static/linux-static/_Concurrency.swiftmodule
5. Run multiple times to observe nondeterministic behavior (sometimes succeeds, sometimes fails with wrong architecture)

Swift Package Manager version/commit hash

No response

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

Swift version: Apple Swift version 6.2 (swift-6.2-RELEASE)
Platform: macOS 26.0.1 (arm64-apple-macosx26.0)
Swift SDK: swift-6.2-RELEASE_static-linux-0.0.1 (multi-arch: x86_64 and aarch64)

[finagolfin]

Heh, I was just about to report this long-standing non-deterministic bug, only to see you beat me to it. 😸 Searching for previous reports, it seems this is just a consequence of #7330 and #7973, ie this behavior has long been known to be broken.

The workaround for now is to run swift build --swift-sdk aarch64-swift-linux-musl instead, which will work provided you don't have multiple SDK bundles installed that provide the same triple.