Runtime crash when checking protocol conformance (`is`) of a global-actor-isolated conformance type whose actor is `final`

[jrsaruo]

Description

When using is to check protocol conformance of a type that has global-actor-isolated conformance whose actor type is declared final, the program crashes at runtime.

This also occurs when using as to cast a type.

Reproduction

@globalActor
final actor FooActor { // IMPORTANT: **final** actor
    static let shared = FooActor()
}

protocol P {}
struct Foo: @FooActor P {}

func checkConformance() {
    let foo = Foo()
    let _ = foo is P // 💥 Runtime crash: EXC_BAD_ACCESS (code=1, address=0x0)
}

checkConformance()

Stack dump

Stack dump:
0.	Program arguments: /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -interpret IsolatedConformanceChecking.swift -Xllvm -aarch64-use-tbi -enable-objc-interop -stack-check -sdk /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.0.sdk -color-diagnostics -Xcc -fcolor-diagnostics -new-driver-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-driver -empty-abi-descriptor -no-auto-bridging-header-chaining -module-name IsolatedConformanceChecking -disable-clang-spi -target-sdk-version 26.0 -target-sdk-name macosx26.0 -external-plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode-26.0.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -in-process-plugin-server-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/libSwiftInProcPluginServer.dylib -plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/local/lib/swift/host/plugins
1.	Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
2.	Compiling with effective version 5.10
3.	While running user code "IsolatedConformanceChecking.swift"
 #0 0x0000000107519bcc (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1066f9bcc)
 #1 0x000000010751755c (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1066f755c)
 #2 0x000000010751a1f8 (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1066fa1f8)
 #3 0x000000019918e744 (/usr/lib/system/libsystem_platform.dylib+0x1804d6744)
 #4 0x00000001ac2f5218 (/usr/lib/swift/libswiftCore.dylib+0x19363d218)
 #5 0x00000001ac2ed174 (/usr/lib/swift/libswiftCore.dylib+0x193635174)
 #6 0x00000001ac2ea668 (/usr/lib/swift/libswiftCore.dylib+0x193632668)
 #7 0x0000000291845c98 (/usr/lib/swift/libswift_Concurrency.dylib+0x278b8dc98)
 #8 0x000000010b88440c 
 #9 0x0000000291847be4 (/usr/lib/swift/libswift_Concurrency.dylib+0x278b8fbe4)
#10 0x00000001ac3269b8 (/usr/lib/swift/libswiftCore.dylib+0x19366e9b8)
#11 0x00000001ac2b90b0 (/usr/lib/swift/libswiftCore.dylib+0x1936010b0)
#12 0x00000001ac2c169c (/usr/lib/swift/libswiftCore.dylib+0x19360969c)
#13 0x00000001ac2bf150 (/usr/lib/swift/libswiftCore.dylib+0x193607150)
#14 0x00000001ac2beccc (/usr/lib/swift/libswiftCore.dylib+0x193606ccc)
#15 0x000000010b884268 
#16 0x000000010b8840ac 
#17 0x000000029187efd0 (/usr/lib/swift/libswift_Concurrency.dylib+0x278bc6fd0)
#18 0x00000002918803c4 (/usr/lib/swift/libswift_Concurrency.dylib+0x278bc83c4)
#19 0x000000010b884074 
#20 0x00000001012bedb8 (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x10049edb8)
#21 0x00000001011d14d0 (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1003b14d0)
#22 0x00000001011de9c4 (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1003be9c4)
#23 0x0000000101158e80 (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x100338e80)
#24 0x00000001011546cc (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1003346cc)
#25 0x0000000101153dbc (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x100333dbc)
#26 0x00000001011576dc (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1003376dc)
#27 0x000000010115568c (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x10033568c)
#28 0x00000001010d6c6c (/Applications/Xcode-26.0.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend+0x1002b6c6c)
#29 0x0000000198dc5d54 
zsh: segmentation fault  swift IsolatedConformanceChecking.swift

Expected behavior

The program should not crash at runtime.
foo is P should be evaluated as:

• true inside the FooActor-isolated context
• false outside the FooActor-isolated context

Environment

swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx26.0

Additional information

Non-crashing samples

FooActor is not final

@globalActor
actor FooActor { // IMPORTANT: **non-final** actor
    static let shared = FooActor()
}

protocol P {}
struct Foo: @FooActor P {}

func checkConformance() {
    let foo = Foo()
    let _ = foo is P // ✅ No crash
}

checkConformance()

Conformance checking on FooActor-isolated context

@globalActor
final actor FooActor { // IMPORTANT: **final** actor
    static let shared = FooActor()
}

protocol P {}
struct Foo: @FooActor P {}

@FooActor // IMPORTANT: isolate to FooActor
func checkConformance() {
    let foo = Foo()
    let _ = foo is P // ✅ No crash
}

await checkConformance()

Related proposal

https://github.com/swiftlang/swift-evolution/blob/main/proposals/0470-isolated-conformances.md

[jamieQ]

hmm, this is not really directly related to the reported crash, but per the documented grammar (search for actor-declaration on the page; contrast with class-declaration) i don't think you're supposed to be able to mark an actor as final as they do not support inheritance (at least not currently).

cc @rintaro @hamishknight – do either of you know if accepting that is intentional? it would be a source break to reject it as it's seemingly been accepted since actors were introduced. unfortunately it also looks like there's a fair amount of OSS code that uses the pattern, so erroring on it in the current language modes may not be plausible, but maybe it should at least be a warning?

---

edit: see discussion here for more info about why this is and why it may not be easy to remove: https://forums.swift.org/t/contradictory-nonisolated-actor-foo-syntax/82905/10

[jrsaruo]

@jamieQ (cc @rintaro @hamishknight )
Thanks for catching.
I see — indeed, according to the grammar, it seems that being able to write final actor at all is actually incorrect.

According to the proposal, actor inheritance might be considered in the future, so personally I think it would be better not to make this a compile-time error — mainly to avoid breaking existing source code.
A warning might make sense, though — similar to how writing await try (instead of try await) only triggers a warning rather than an error.

That said, whichever decision is made, it’ll likely be a reasonable one, and I’m happy to accept it either way.

[hamishknight]

I don't believe allowing final actor was intentional, emitting a warning for it seems reasonable to me

[jamieQ]

in addition to removing the final attribute, it looks like if the sharedUnownedExecutor method is implemented on the actor (even if final) it will avoid the runtime crash (not that that's really a desirable workaround). it's not yet clear to me where the ultimate issue may be exactly... the intermediate output of the final vs non-final cases looked essentially identical to me. FWIW i tested the repro on macos 15.6.1 and it did not crash... cc @ktoso does the backtrace in the crash give you an ideas as to what may be going on here?

---

edit: oops, would help if i'd included the symbolized stack (from this example):

*** Program crashed: Bad pointer dereference at 0x0000000000000000 ***

Platform: x86_64 Linux (Ubuntu 22.04.5 LTS)

Thread 0 "output.s" crashed:

  0              0x000071854fa1890d swift::TargetMetadata<swift::InProcess>::isCanonicalStaticallySpecializedGenericMetadata() const + 189 in libswiftCore.so
  1 [ra]         0x000071854fa2694c swift::MetadataResponse performOnMetadataCache<swift::MetadataResponse, swift_checkMetadataState::CheckStateCallbacks>(swift::TargetMetadata<swift::InProcess> const*, swift_checkMetadataState::CheckStateCallbacks&&) + 27 in libswiftCore.so
  2 [ra]         0x000071854fa2691b swift_checkMetadataState + 26 in libswiftCore.so
  3 [ra]         0x000071854fa55111 (anonymous namespace)::ConformanceCandidate::getMatchingType(swift::TargetMetadata<swift::InProcess> const*, bool) const + 288 in libswiftCore.so
  4 [ra]         0x000071854fa54efe swift::findConformingSuperclass(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolConformanceDescriptor<swift::InProcess> const*) + 45 in libswiftCore.so
  5 [ra]         0x000071854fa26042 swift_getAssociatedTypeWitnessSlow + 241 in libswiftCore.so
  6 [ra]         0x000071854fa25f47 swift_getAssociatedTypeWitness + 38 in libswiftCore.so
  7 [ra]         0x000071854f58766b static GlobalActor.sharedUnownedExecutor.getter + 58 in libswift_Concurrency.so
  8 [ra] [thunk] 0x000062ce4ebc453c protocol witness for static GlobalActor.sharedUnownedExecutor.getter in conformance FooActor + 11 in output.s at //<compiler-generated>
  9 [ra]         0x000071854f55b9a7 _swift_task_isCurrentGlobalActor + 6 in libswift_Concurrency.so
 10 [ra]         0x000071854fa554e7 swift_isInConformanceExecutionContext + 38 in libswiftCore.so
 11 [ra]         0x000071854f9ff619 swift::_conformsToProtocolInContext(swift::OpaqueValue const*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptorRef<swift::InProcess>, swift::TargetWitnessTable<swift::InProcess> const**, bool) + 104 in libswiftCore.so
 12 [ra]         0x000071854fa04a87 tryCastToConstrainedOpaqueExistential(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetMetadata<swift::InProcess> const*&, swift::TargetMetadata<swift::InProcess> const*&, bool, bool, bool) + 198 in libswiftCore.so
 13 [ra]         0x000071854fa03163 tryCast(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetMetadata<swift::InProcess> const*&, swift::TargetMetadata<swift::InProcess> const*&, bool, bool, bool) + 642 in libswiftCore.so
 14 [ra]         0x000071854fa02e79 swift_dynamicCast + 88 in libswiftCore.so
 15 [ra]         0x000062ce4ebc4226 checkConformance() + 53 in output.s at /app/example.swift:11:17
 16 [ra]         0x000062ce4ebc4459 main + 8 in output.s at /app/example.swift:14:1
...