SIL ownership/lifetime verifier crash on `catch` cast to the function's typed-`throws` error type (Windows, Swift 6.2)

[broken-circle]

Description

Compiling a Swift package on Windows with the Swift 6.2.0 toolchain crashes swift-frontend during SIL ownership verification. The crash is Found outside of lifetime use?! followed by Found ownership error?!. The same source compiles without crashing on macOS and Linux.

Reproduction

The crash occurs while compiling Configuration.swift in swiftlang/swift-subprocess#261. The relevant function and the catch block that triggers it:

internal func run<Result, Input: InputProtocol, Output: OutputProtocol, Error: OutputProtocol>(
    input: consuming CreatedPipe,
    as inputType: Input.Type,
    output: consuming CreatedPipe,
    as outputType: Output.Type,
    error: consuming CreatedPipe,
    as errorType: Error.Type,
    _ body: (
        (
            ProcessIdentifier,
            consuming IODescriptor?,
            consuming IODescriptor?,
            consuming IODescriptor?
        ) async throws -> Result
    )
) async throws -> ExecutionOutcome<Result> {
    let baseContext = ExecutionContext(self)

    var spawnResults: SpawnResult
    do {
        spawnResults = try await self.spawn(
            withInput: input,
            outputPipe: output,
            errorPipe: error
        )
    } catch let error as SubprocessError { // warning emitted + crash
        throw error.withExecutionContext(baseContext)
    }

    // ...
}

withExecutionContext(_:) returns SubprocessError. On Windows, spawn(withInput:outputPipe:errorPipe:) uses throws(SubprocessError); on other platforms it is an untyped throw.

The workaround is to branch on the OS:

var spawnResults: SpawnResult
#if os(Windows)
do {
    spawnResults = try await self.spawn(
        withInput: input,
        outputPipe: output,
        errorPipe: error
    )
} catch {
    throw error.withExecutionContext(baseContext)
}
#else
do {
    spawnResults = try await self.spawn(
        withInput: input,
        outputPipe: output,
        errorPipe: error
    )
} catch let error as SubprocessError {
    throw error.withExecutionContext(baseContext)
}
#endif

Stack dump

Full stack dump and program arguments

error: compile command failed due to exception 3 (use -v to see invocation)
Begin Error in Function: '$s10Subprocess13ConfigurationV3run5input2as6outputAF5errorAF_AA16ExecutionOutcomeVyxGAA11CreatedPipeVn_q_mAMnq0_mAMnq1_mxAA17ProcessIdentifierV_AA12IODescriptorVSgnARnARntYaKYCXEtYaKs8SendableRzAA13InputProtocolR_AA06OutputQ0R0_AaUR1_r2_lF'

Found outside of lifetime use?!

Value: %133 = argument of bb4 : $SubprocessError         // users: %138, %169, %178

Consuming User:   store %133 to [init] %176 : $*SubprocessError   // id: %178

Non Consuming User:   end_borrow %138 : $SubprocessError              // id: %184

Block: bb6



End Error in Function: '$s10Subprocess13ConfigurationV3run5input2as6outputAF5errorAF_AA16ExecutionOutcomeVyxGAA11CreatedPipeVn_q_mAMnq0_mAMnq1_mxAA17ProcessIdentifierV_AA12IODescriptorVSgnARnARntYaKYCXEtYaKs8SendableRzAA13InputProtocolR_AA06OutputQ0R0_AaUR1_r2_lF'

Found ownership error?!

D:\a\swift-subprocess-test\swift-subprocess-test\Sources\Subprocess\Configuration.swift:112:27: warning: 'as' test is always true

 110 |                 errorPipe: error

 111 |             )

 112 |         } catch let error as SubprocessError {

     |                           `- warning: 'as' test is always true

 113 |             throw error.withExecutionContext(baseContext)

 114 |         }



<unknown>:0: error: fatal error encountered during compilation; please submit a bug report (https://swift.org/contributing/#reporting-bugs)

<unknown>:0: note: triggering standard assertion failure routine

Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.

Stack dump:

0.	Program arguments: C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\bin\\swift-frontend.exe -frontend -c -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\API.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Buffer.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Configuration.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Error.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Execution.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\ExecutionContext.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\IO\\AsyncIO+KQueue.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\IO\\AsyncIO+Linux.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\IO\\AsyncIO+Unix.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\IO\\AsyncIO+Windows.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\IO\\Input.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\IO\\Output.swift -primary-file D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Platforms\\Subprocess+BSD.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Platforms\\Subprocess+Darwin.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Platforms\\Subprocess+Linux.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Platforms\\Subprocess+Unix.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Platforms\\Subprocess+Windows.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Result.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Span+Subprocess.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\SubprocessFoundation\\Input+Foundation.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\SubprocessFoundation\\Output+Foundation.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\SubprocessFoundation\\Span+SubprocessFoundation.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\SubprocessOutputSequence.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Teardown.swift D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\Subprocess\\Thread.swift -supplementary-output-file-map C:\\Users\\runneradmin\\AppData\\Local\\Temp\\TemporaryDirectory.1ObWvc\\supplementaryOutputs-1 -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\SDKs\\Windows.sdk\\ -I D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Modules -I C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\Library\\XCTest-6.2.0\\usr\\lib\\swift\\windows -I C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\Library\\XCTest-6.2.0\\usr\\lib\\swift\\windows\\x86_64 -I C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\Library\\Testing-6.2.0\\usr\\lib\\swift\\windows -no-color-diagnostics -Xcc -fno-color-diagnostics -enable-testing -g -debug-info-format=dwarf -dwarf-version=4 -module-cache-path D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\ModuleCache -static -swift-version 6 -Onone -D SWIFT_PACKAGE -D DEBUG -D SWIFT_MODULE_RESOURCE_BUNDLE_UNAVAILABLE -D SubprocessFoundation -enable-experimental-feature StrictConcurrency -enable-experimental-feature Lifetimes -enable-upcoming-feature NonisolatedNonsendingByDefault -enable-upcoming-feature ExistentialAny -enable-upcoming-feature MemberImportVisibility -enable-upcoming-feature InternalImportsByDefault -empty-abi-descriptor -enable-anonymous-context-mangled-names -file-compilation-dir D:\\a\\swift-subprocess-test\\swift-subprocess-test -Xcc -fmodule-map-file=D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\_SubprocessCShims\\include\\module.modulemap -Xcc -I -Xcc D:\\a\\swift-subprocess-test\\swift-subprocess-test\\Sources\\_SubprocessCShims\\include -Xcc -fmodule-map-file=D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\checkouts\\swift-system\\Sources\\CSystem\\include\\module.modulemap -Xcc -I -Xcc D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\checkouts\\swift-system\\Sources\\CSystem\\include -Xcc -D_MT -Xcc -D_DLL -Xcc -Xclang -Xcc --dependent-lib=msvcrt -Xcc -gdwarf -no-auto-bridging-header-chaining -module-name Subprocess -package-name swift_subprocess_test -in-process-plugin-server-path C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\bin\\SwiftInProcPluginServer.dll -plugin-path C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\bin -plugin-path C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\local\\bin -autolink-library oldnames -autolink-library msvcrt -Xcc -D_MT -Xcc -D_DLL -parse-as-library -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\API.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Buffer.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Configuration.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Error.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Execution.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\ExecutionContext.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\AsyncIO+KQueue.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\AsyncIO+Linux.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\AsyncIO+Unix.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\AsyncIO+Windows.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Input.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Output.swift.o -o D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\Subprocess.build\\Subprocess+BSD.swift.o -index-store-path D:\\a\\swift-subprocess-test\\swift-subprocess-test\\.build\\x86_64-unknown-windows-msvc\\debug\\index\\store -index-system-modules

#17 0x00007ff6b49ed60d (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x172d60d)

#18 0x00007ff6b401b5d9 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd5b5d9)

#19 0x00007ff6b401215d (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd5215d)

#20 0x00007ff6b4014eab (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd54eab)

#21 0x00007ff6b40112ef (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd512ef)

#22 0x00007ff6b405f248 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd9f248)

#23 0x00007ff6b4059c1d (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd99c1d)

#24 0x00007ff6b405f3f8 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd9f3f8)

#25 0x00007ff6b401ca15 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd5ca15)

#26 0x00007ff6b4015105 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd55105)

#27 0x00007ff6b40157e6 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd557e6)

#28 0x00007ff6b401ce16 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd5ce16)

#29 0x00007ff6b400b773 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd4b773)

#30 0x00007ff6b401b2c4 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0xd5b2c4)

#31 0x00007ff6b3907d69 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x647d69)

#32 0x00007ff6b3906b34 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x646b34)

#33 0x00007ff6b3906feb (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x646feb)

#34 0x00007ff6b3908f52 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x648f52)

#35 0x00007ff6b3754110 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x494110)

#36 0x00007ff6b3753ca7 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x493ca7)

#37 0x00007ff6ba226698 (C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe+0x6f66698)

#38 0x00007ffb7165e8d7 (C:\Windows\System32\KERNEL32.DLL+0x2e8d7)

#39 0x00007ffb726cc53c (C:\Windows\SYSTEM32\ntdll.dll+0x8c53c)

Expected behavior

The compiler should either compile the code successfully or emit an error diagnostic, not crash with an internal SIL verification failure.

Environment

• swift-6.2-RELEASE, asserts-enabled toolchain
• Target: x86_64-unknown-windows-msvc, -Onone, -swift-version 6
• GitHub Actions windows-latest

Additional information

No response

[heckj]

I just ran into this same issue myself today - fiddling with typed throws in tests on a side project (Mac/Linux) and running into issues with the #expect macro. I don't think this is just windows - I hit the same issue when catching a typed error.

I used claude to drive out a minimal reproduction:
Two files and swift test (or swift build --build-tests) crashes swift-frontend.

Package.swift

// swift-tools-version: 6.3
import PackageDescription

let package = Package(
    name: "TypedCatchCrash",
    targets: [
        .testTarget(name: "TypedCatchCrashTests"),
    ],
    swiftLanguageModes: [.v6]
)

Tests/TypedCatchCrashTests/ReproTests.swift

import Testing

// A non-trivial Error: it owns a String, so SIL must manage its ownership.
// An all-trivial payload (e.g. all-Int) does NOT reproduce the crash.
struct MyError: Error {
    let code: Int
    let message: String
}

// Typed throw: the thrown error type is fixed via `throws(MyError)`.
// An untyped `throws` does NOT reproduce the crash.
func throwsMyError() throws(MyError) {
    throw MyError(code: 7, message: "boom")
}

@Test func typedCatchUsedAcrossTwoExpectsCrashes() throws {
    do {
        try throwsMyError()
        Issue.record("expected throwsMyError() to throw")
    } catch let error as MyError {
        #expect(error.code == 7)
        #expect(error.message.contains("boom"))
    }
}

This dropped a SIL signature that matches this issue — SILGenCleanup emits a consuming store [init] of the caught error alongside an end_borrow of the same value:

Begin Error in Function: '...typedCatchUsedAcrossTwoExpectsCrashes...'
Found outside of lifetime use?!
Value: %61 = argument of bb3 : $MyError
Consuming User:     store %61 to [init] %298 : $*MyError
Non Consuming User: end_borrow %62 : $MyError
Found ownership error?!
...
While running pass #0 SILModuleTransform "SILGenCleanup".

Required ingredients (drop any one and it compiles cleanly), verified by toggling each:

1. a typed throw (throws(MyError)) caught with catch let error as MyError — an untyped throws is fine;
2. a non-trivial error payload (owns a String) — an all-Int payload is fine;
3. the caught error is used inside #expect(...) macro expansions.

And it seems to have regressed slightly on main vs. 6.3.2 where I was able to copy the error to work around this

case	swift-6.3.2-RELEASE	main (6.5-dev snapshot)
two #expect (above)	❌ crash	❌ crash
single #expect	✅ clean	❌ crash
copy error into a local var first, then assert	✅ clean	❌ crash
all-Int payload	✅ clean	✅ clean
untyped throws	✅ clean	✅ clean

• Apple Swift 6.3.1 / 6.3.2-RELEASE, arm64-apple-macosx
• swift-DEVELOPMENT-SNAPSHOT-2026-05-18-a (Swift 6.5-dev, +assertions), arm64-apple-macosx
• swiftlang/swift:nightly-main-noble (Swift 6.5-dev, +assertions), aarch64-unknown-linux-gnu

[heckj]

/cc @grynspan since I ran into this using #expect in testing - I thought originally it might have been an issue with the macro, but found this...