Generated test harness TestRunner.swift fails to compile under Swift 6 strict concurrency

[SeraphimSakuja]

Environment

skip (Skip.git)	1.9.7 (26808ac3604663a73ad79cbacccb287d6a3957d4)
Xcode	26.4.1 (17E202)
Toolchain for the Android SDK build	swift-6.2.3-RELEASE.xctoolchain (chosen by Skip itself, --swift-sdk aarch64-unknown-linux-android28)
Affected paths	skip android test --apk and the instrumented XCSkipTests path (ANDROID_SERIAL → connectedDebugAndroidTest)
Project	Swift Package Manager multiplatform app using Skip Fuse

Description

Both test execution paths generate a temporary test harness
(.../T/<uuid>/harness/Sources/TestHarness/TestRunner.swift), whose line 154 fails to compile
under Swift 6 strict concurrency:

152 |     let semaphore = DispatchSemaphore(value: 0)
153 |     nonisolated(unsafe) var testSuccess = false
154 |     Task {
    |     |- error: sending value of non-Sendable type '() async -> ()' risks causing data races [#SendingRisksDataRace]
    |     `- note: Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to initializer 'init(name:priority:operation:)' risks causing races in between local and caller code
155 |         defer { semaphore.signal() }
156 |         do {
    :
162 |     semaphore.wait()
163 |
164 |     let exitCode: Int32 = testSuccess ? 0 : 1
    |                           `- note: access can happen concurrently
165 |     return jint(exitCode)
166 | }

TestRunner.swift is generated by Skip itself — there is no way to influence the generated
harness via project or compiler flags. -Xswiftc -swift-version -Xswiftc 5 in our own
Package.swift does not reach this build, because Skip builds the harness in its own temporary
package with its own flags.

Before upgrading to Skip 1.9.7, a separate Kotlin generator defect (Unresolved reference 'Bundle') masked the instrumented path — that defect is fixed as of this upgrade, and
TestRunner.swift:154 is now the same visible blocker on both paths.

Reproduction

1. skip android test --apk log

[0/1] Planning build
[1/1] Compiling plugin Create SkipLink
[2/2] Compiling plugin skipstone
Building for debugging...
[2/16] Write swift-version--35B2706C3EC7C88B.txt
Build complete! (8.89s)
[✓] Check Swift Package (0.64s)
Building for debugging...
[0/5] Write sources
[1/5] Write swift-version--35B2706C3EC7C88B.txt
error: emit-module command failed with exit code 1 (use -v to see invocation)
[3/7] Emitting module TestHarness
/private/var/.../T/7CC6111F-E935-4B29-97B8-E0DC8A7641EC/harness/Sources/TestHarness/TestRunner.swift:154:5: error: sending value of non-Sendable type '() async -> ()' risks causing data races [#SendingRisksDataRace]
152 |     let semaphore = DispatchSemaphore(value: 0)
153 |     nonisolated(unsafe) var testSuccess = false
154 |     Task {
    |     |- error: sending value of non-Sendable type '() async -> ()' risks causing data races [#SendingRisksDataRace]
    |     `- note: Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to initializer 'init(name:priority:operation:)' risks causing races in between local and caller code
155 |         defer { semaphore.signal() }
156 |         do {
    :
162 |     semaphore.wait()
163 |
164 |     let exitCode: Int32 = testSuccess ? 0 : 1
    |                           `- note: access can happen concurrently
165 |     return jint(exitCode)
166 | }

[#SendingRisksDataRace]: <https://docs.swift.org/compiler/documentation/diagnostics/sending-risks-data-race>
Error: Error terminated(code: 1) running command: ~/Library/Developer/Toolchains/swift-6.2.3-RELEASE.xctoolchain/usr/bin/swift build --swift-sdk aarch64-unknown-linux-android28 --package-path .../harness --configuration debug

2. Instrumented XCSkipTests / connectedDebugAndroidTest log

[2/2] Compiling plugin skipstone
Building for debugging...
[2/18] Write swift-version--35B2706C3EC7C88B.txt
Build complete! (5.13s)
[✓] Check Swift Package (0.15s)
Building for debugging...
[0/5] Write sources
[1/5] Write swift-version--35B2706C3EC7C88B.txt
error: emit-module .../checkouts/skip/Sources/SkipTest/XCGradleHarness.swift:356: error: -[XCSkipTests testSkipModule] : error: emit-module command failed with exit code 1 (use -v to see invocation)
.../checkouts/skip/Sources/SkipTest/XCGradleHarness.swift:356: error: -[XCSkipTests testSkipModule] : Execution failed for task ':buildAndroidSwiftTestLibs' (registered in build file 'build.gradle.kts').
.../plugins/outputs/.../SkipTestHarness/XCSkipTests.swift:10: error: -[XCSkipTests testSkipModule] : failed - The expected test output folder did not exist, which may indicate that the gradle process encountered a build error or other issue.
Test Case '-[XCSkipTests testSkipModule]' failed (30.856 seconds).
[3/7] Emitting module TestHarness
.../T/80D22003-B19A-4C60-B411-D6A62A705B1A/harness/Sources/TestHarness/TestRunner.swift:154:5: error: sending value of non-Sendable type '() async -> ()' risks causing data races [#SendingRisksDataRace]
152 |     let semaphore = DispatchSemaphore(value: 0)
153 |     nonisolated(unsafe) var testSuccess = false
154 |     Task {
    |     |- error: sending value of non-Sendable type '() async -> ()' risks causing data races [#SendingRisksDataRace]
    |     `- note: Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to initializer 'init(name:priority:operation:)' risks causing races in between local and caller code
155 |         defer { semaphore.signal() }
156 |         do {
    :
162 |     semaphore.wait()
163 |
164 |     let exitCode: Int32 = testSuccess ? 0 : 1
    |                           `- note: access can happen concurrently
165 |     return jint(exitCode)
166 | }

[#SendingRisksDataRace]: <https://docs.swift.org/compiler/documentation/diagnostics/sending-risks-data-race>
Error: Error terminated(code: 1) running command: ~/Library/Developer/Toolchains/swift-6.2.3-RELEASE.xctoolchain/usr/bin/swift build --package-path .../harness --configuration debug --swift-sdk aarch64-unknown-linux-android28 --build-system native

FAILURE: Build failed with an exception.

Both logs show the same failing line in the generated TestRunner.swift:154.

Expected vs. Actual

• Expected: The generated test harness compiles under Swift 6 strict concurrency.
• Actual: emit-module aborts with #SendingRisksDataRace, blocking both test paths.

Impact

Without a fix, the instrumented Android test lane (with a real Looper / main-thread
semantics) is unreachable, which blocks automated verification of @MainActor/main-thread
behavior that today can only be checked via manual screenshots. The regular skip android test
lane (non-instrumented) is not affected — it uses a different path.