DistributedActor as Actor witness already known crash in DistributedCluster library

[ktoso]

This is not a "still" but a new issue / regression in 6.0, so making a new issue for it.

We're working on it actively actually, it is a 6.0 regression.

Radar tracking: rdar://127206143

---

          Reproducer still crashes with the 05/24 toolchain. Stack trace looks a bit different from the original issue.

4.	While evaluating request ASTLoweringRequest(Lowering AST to SIL for file "/Users/kshitij/workspace/scratch/swift-distributed-actors/Sources/DistributedCluster/Plugins/ClusterSingleton/ClusterSingletonBoss.swift")
5.	While generating SIL witness table protocol conformance Self: Actor at extension of DistributedActor (in module 'Distributed')
6.	While reading from module 'Distributed', builder version '6.0(5.10)/Apple Swift version 6.0-dev (LLVM b66077aefd3be08, Swift 84d36181a762913)', built from swiftinterface, resilient, loaded from '/Users/kshitij/workspace/scratch/swift-distributed-actors/Samples/.build/arm64-apple-macosx/debug/ModuleCache/Distributed-3DIVDCQGHH6N9.swiftmodule'
7.	While finishing conformance for protocol conformance Self: Actor at extension of DistributedActor (in module 'Distributed')
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  swift-frontend           0x0000000105c91600 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x0000000105c8fdcc llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x0000000105c91c48 SignalHandler(int) + 304
3  libsystem_platform.dylib 0x00000001a3286a24 _sigtramp + 56
4  libsystem_pthread.dylib  0x00000001a3257c28 pthread_kill + 288
5  libsystem_c.dylib        0x00000001a3165ae8 abort + 180
6  libsystem_c.dylib        0x00000001a3164e44 err + 0
7  swift-frontend           0x00000001061da454 swift::NormalProtocolConformance::setWitness(swift::ValueDecl*, swift::Witness) const (.cold.7) + 0
8  swift-frontend           0x000000010207f6bc swift::NormalProtocolConformance::setWitness(swift::ValueDecl*, swift::Witness) const + 192
9  swift-frontend           0x0000000101519d34 swift::ModuleFile::finishNormalConformance(swift::NormalProtocolConformance*, unsigned long long) + 3840
10 swift-frontend           0x000000010207da58 swift::NormalProtocolConformance::getWitness(swift::ValueDecl*) const + 88
11 swift-frontend           0x0000000100f3e2c4 (anonymous namespace)::SILGenWitnessTable<(anonymous namespace)::SILGenConformance>::addMethod(swift::SILDeclRef) + 128
12 swift-frontend           0x0000000100f3e1c0 void llvm::function_ref<void (swift::AccessorDecl*)>::callback_fn<swift::SILWitnessVisitor<(anonymous namespace)::SILGenConformance>::visitAbstractStorageDecl(swift::AbstractStorageDecl*)::'lambda'(swift::AccessorDecl*)>(long, swift::AccessorDecl*) + 132
13 swift-frontend           0x0000000101ec0074 swift::AbstractStorageDecl::visitExpectedOpaqueAccessors(llvm::function_ref<void (swift::AccessorKind)>) const + 68
14 swift-frontend           0x0000000101ebfe9c swift::AbstractStorageDecl::visitOpaqueAccessors(llvm::function_ref<void (swift::AccessorDecl*)>) const + 40
15 swift-frontend           0x0000000100f3dee8 swift::SILWitnessVisitor<(anonymous namespace)::SILGenConformance>::visitProtocolDecl(swift::ProtocolDecl*) + 1800
16 swift-frontend           0x0000000100f3b6b0 swift::Lowering::SILGenModule::getWitnessTable(swift::NormalProtocolConformance*) + 324
17 swift-frontend           0x0000000100e42920 swift::ASTLoweringRequest::evaluate(swift::Evaluator&, swift::ASTLoweringDescriptor) const + 2120
18 swift-frontend           0x0000000100f2aecc swift::SimpleRequest<swift::ASTLoweringRequest, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>> (swift::ASTLoweringDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::ASTLoweringRequest const&, swift::Evaluator&) + 148
19 swift-frontend           0x0000000100e47d3c swift::ASTLoweringRequest::OutputType swift::Evaluator::getResultUncached<swift::ASTLoweringRequest, swift::ASTLoweringRequest::OutputType swift::evaluateOrFatal<swift::ASTLoweringRequest>(swift::Evaluator&, swift::ASTLoweringRequest)::'lambda'()>(swift::ASTLoweringRequest const&, swift::ASTLoweringRequest::OutputType swift::evaluateOrFatal<swift::ASTLoweringRequest>(swift::Evaluator&, swift::ASTLoweringRequest)::'lambda'()) + 340
20 swift-frontend           0x0000000100e42d4c swift::performASTLowering(swift::FileUnit&, swift::Lowering::TypeConverter&, swift::SILOptions const&, swift::IRGenOptions const*) + 96
21 swift-frontend           0x000000010082c22c swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 568
22 swift-frontend           0x000000010083d4cc withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 160
23 swift-frontend           0x000000010082e868 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 708
24 swift-frontend           0x000000010082d7d4 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2368
25 swift-frontend           0x00000001006489cc swift::mainEntry(int, char const**) + 3096
26 dyld                     0x00000001a2efff28 start + 2236

Originally posted by @jkshtj in #62898 (comment)

[ktoso]

Incoming fix #73781

[ktoso]

For Apple tracking, this is a dupe of rdar://127206143 and fix is incoming ^^^