`'Data' has no member 'bytes'` build error on macos-14 GHA runners

[rgoldberg]

Describe the bug

On macos-14 GHA runners, building swift-subprocess 1.0.0 as a dependency for a SwiftPM 6.3 project reports the following error (I can supply the log, if necessary):

.build/checkouts/swift-subprocess/Sources/Subprocess/SubprocessFoundation/Input+Foundation.swift:154:42: error: value of type 'Data' has no member 'bytes'

swift-subprocess 0.5.0 does not produce this error.

To Reproduce

I imagine the error should occur when building on any macOS 14.

I set up the Swift 6.3 toolchain using:

swiftly init -ny
swiftly link latest

Building on macos-26 & macos-26-intel GHA runners did not output any errors.

I don't know about macos-15 & macos-15-intel, because the macos-14 failure canceled those before they finished.

I do not get any errors building with swift from Xcode 26.6 on my macOS 26.6.1 M4 Max Studio.

Expected behavior

No build error.

Environment (please complete the following information):

• OS [e.g. macOS 15]: macOS 14.8.7 (on macos-14 GHA runner)
• Swift version (run swift --version): Swift 6.3.3 installed via Swiftly

Additional context

Using Data.withUnsafeBytes(_:) instead of Data.bytes should solve this.

[broken-circle]

Thanks for reporting this. The error is inside Subprocess's own sources, so nothing in your package's source or settings can affect it.

On Darwin, Foundation comes from the SDK in the active Xcode rather than from the toolchain swiftly installs, and Data.bytes requires the macOS 26 SDK. On a macos-15 runner with the same swiftly 6.3.3 toolchain, Xcode 16.4 (macOS 15.5 SDK) reproduces your error and Xcode 26.0.1 (macOS 26.0 SDK) builds clean, so DEVELOPER_DIR=/Applications/Xcode_26.3.app/Contents/Developer unblocks macos-15. macos-14 tops out at Xcode 16.2 (macOS 15.2 SDK), older than the 15.5 SDK shown to fail, so nothing on that image will build it. The accessor back-deploys, so the requirement is build-time only and your deployment target is unaffected.

Data.withUnsafeBytes(_:) isn't a drop-in substitute as "Additional context" suggests, since the write is async and the closure is synchronous and non-escaping.

On a related note, the macOS 14 runner images were deprecated on July 6th and will be unsupported on November 2nd.

[broken-circle]

#364 adds the missing rows and documents the SDK requirement, so I'm closing this. The workaround above is still what unblocks a build on an older Xcode: selecting Xcode 26 through DEVELOPER_DIR, or moving off macos-14, which is unsupported after November 2nd.