nix/var/nix/builds/nix-build-swift-6.2.1.drv-0/b/swift/stdlib/public/core/SwiftNativeNSArray.swift:420:3: error: missing return in instance method expected to return 'R'
  }
  ^
/nix/var/nix/builds/nix-build-swift-6.2.1.drv-0/b/swift/stdlib/public/core/SwiftNativeNSArray.swift:474:3: error: missing return in instance method expected to return 'R'
  }
  ^

I applied a similar patch and got further.

swift> 1.       Swift version 6.2.1 (swift-6.2.1-RELEASE)
swift> 2.       Compiling with effective version 5.10
swift> 3.       While evaluating request ExecuteSILPipelineRequest(Run pipelines { PrepareOptimizationPasses, EarlyModulePasses, HighLevel,Function+EarlyLoopOpt, HighLevel,Module+StackPromote, MidLevel,Function, ClosureSpecialize, LowLevel,Function, LateLoopOpt, SIL Debug Info Generator } on SIL for _Concurrency)
swift> 4.       While running pass #509344 SILFunctionTransform "CodeSinking" on SILFunction "@$ss6_DequeV8_StorageV13_growCapacity2to8linearlyS2i_SbtFyt_Tg5".
swift>  for '_growCapacity(to:linearly:)' (at /nix/var/nix/builds/nix-build-swift-6.2.1.drv-0/b/swift/stdlib/public/Concurrency/Deque/Deque+Storage.swift:134:12)
swift> 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):
swift> 0  libLLVM.dylib            0x000000011bcc085c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 72
swift> 1  libLLVM.dylib            0x000000011bcbe5a0 llvm::sys::RunSignalHandlers() + 108
swift> 2  libLLVM.dylib            0x000000011bcc0f38 SignalHandler(int, __siginfo*, void*) + 324
swift> 3  libsystem_platform.dylib 0x000000019e9cb744 _sigtramp + 56
swift> 4  swift-frontend           0x00000001089b130c (anonymous namespace)::CodeSinkingPass::sinkInstruction(swift::SILInstruction*) + 496
swift> 5  swift-frontend           0x00000001089b130c (anonymous namespace)::CodeSinkingPass::sinkInstruction(swift::SILInstruction*) + 496
swift> 6  swift-frontend           0x00000001089b1048 (anonymous namespace)::CodeSinkingPass::run() + 264
swift> 7  swift-frontend           0x00000001088a03b4 swift::SILPassManager::runFunctionPasses(unsigned int, unsigned int) + 6224
swift> 8  swift-frontend           0x000000010889db44 swift::SILPassManager::executePassPipelinePlan(swift::SILPassPipelinePlan const&) + 11880
swift> 9  swift-frontend           0x00000001088b880c swift::SimpleRequest<swift::ExecuteSILPipelineRequest, std::__1::tuple<> (swift::SILPipelineExecutionDescriptor), (swift::RequestFlags)1>::evaluateRequest(swift::ExecuteSILPipelineRequest const&, swift::Evaluator&) + 64
swift> 10 swift-frontend           0x00000001088a1638 swift::executePassPipelinePlan(swift::SILModule*, swift::SILPassPipelinePlan const&, bool, swift::irgen::IRGenModule*) + 380
swift> 11 swift-frontend           0x00000001088a4efc swift::runSILOptimizationPasses(swift::SILModule&) + 384
swift> 12 swift-frontend           0x0000000107f68c28 swift::CompilerInstance::performSILProcessing(swift::SILModule*) + 2540
swift> 13 swift-frontend           0x0000000107e10f94 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*) + 1252
swift> 14 swift-frontend           0x0000000107e1020c swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 2336
swift> 15 swift-frontend           0x0000000107e23d90 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 7868
swift> 16 swift-frontend           0x0000000107e2110c swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6440
swift> 17 swift-frontend           0x0000000107da37dc swift::mainEntry(int, char const**) + 5564
swift> 18 dyld                     0x000000019e5f9d54 start + 7184
swift> <unknown>:0: error: unable to execute command: Segmentation fault: 11
swift> <unknown>:0: error: compile command failed due to signal 11 (use -v to see invocation)

$ nix build -f . swiftPackages.swiftc
$ result/bin/swiftc -version
<unknown>:0: warning: using (deprecated) legacy driver, Swift installation does not contain swift-driver at: '/nix/store/qv81mmcyl52jzgy32pnq41kxx6v575j9-swift-6.2.1/bin/swift-driver-new'
Swift version 6.2.1 (swift-6.2.1-RELEASE)
Target: arm64-apple-darwin25.1.0

$ SDKROOT=$(nix eval --raw -f . apple-sdk_26.sdkroot) result/bin/swiftc ../test-cli.swift -o ../test-cli
$ ../test-cli
Hello, Swift!