Swift 6 Compiler Crash When Using Typed Throws in Function Declaration

[nashysolutions]

Description

The Swift 6 compiler crashes when compiling a function that uses typed throws with an error type. Removing the generic error type from the throws clause allows the code to compile successfully.

Reproduction

import Foundation

public struct FileStorage<Path: Codable> {
    
    public func savePath(_ path: Path, storageKey: String) throws(SaveError) {
    }
}

public extension FileStorage {
    
    enum SaveError: Error {
        case encodingFailed
    }
}

Error: Command SwiftCompile failed with nonzero exit code.

However, if I remove the generic error type from the throws clause, the code compiles successfully:

import Foundation

public struct FileStorage {
    
    public func savePath(_ path: String, storageKey: String) throws(SaveError) {
    }
}

public extension FileStorage {
    
    enum SaveError: Error {
        case encodingFailed
    }
}

Expected behavior

Expected behavior

The compiler should not crash. Instead, it should either:
1. Successfully compile the function, or
2. Provide a clear and helpful error message if the syntax is incorrect.

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0

Xcode 16.2
Build version 16C5032a

ProductName: macOS
ProductVersion: 15.2
BuildVersion: 24C101

// swift-tools-version: 6.0

import PackageDescription

let package = Package(
    name: "Something",
    products: [
        .library(
            name: "Something",
            targets: ["Something"]),
    ],
    targets: [
        .target(
            name: "Something"
        )
    ]
)

Additional information

No response

[Lancelotbronner]

Providing a stack dump to help as this may be related to another crash I've encountered with parameter packs and typed throws.

• Parameter packs and passing tuples as arguments crashes Swift #78646
• Parameter pack crash: Can't "untuple" a single parameter pack from a tuple #78671
• Is there a way to spread a tuple into the arguments of a function? (Swift forums)

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0

0.	Program arguments: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -c -primary-file /Users/lancelot/Downloads/test/Sources/main.swift -emit-dependencies-path /Users/lancelot/Downloads/test/.build/arm64-apple-macosx/debug/test.build/main.d -emit-reference-dependencies-path /Users/lancelot/Downloads/test/.build/arm64-apple-macosx/debug/test.build/main.swiftdeps -target arm64-apple-macosx14.0 -Xllvm -aarch64-use-tbi -enable-objc-interop -stack-check -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -I /Users/lancelot/Downloads/test/.build/arm64-apple-macosx/debug/Modules -I /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib -F /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -color-diagnostics -enable-testing -g -debug-info-format=dwarf -dwarf-version=4 -module-cache-path /Users/lancelot/Downloads/test/.build/arm64-apple-macosx/debug/ModuleCache -swift-version 6 -Onone -D SWIFT_PACKAGE -D DEBUG -new-driver-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-driver -entry-point-function-name test_main -empty-abi-descriptor -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir /Users/lancelot/Downloads/test -Xcc -isysroot -Xcc /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -Xcc -F -Xcc /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -Xcc -fPIC -Xcc -g -module-name test -package-name test -disable-clang-spi -target-sdk-version 15.2 -target-sdk-name macosx15.2 -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/local/lib/swift/host/plugins -o /Users/lancelot/Downloads/test/.build/arm64-apple-macosx/debug/test.build/main.swift.o -index-store-path /Users/lancelot/Downloads/test/.build/arm64-apple-macosx/debug/index/store -index-system-modules
1.	Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
2.	Compiling with the current language version
3.	While evaluating request IRGenRequest(IR Generation for file "/Users/lancelot/Downloads/test/Sources/main.swift")
4.	While emitting IR SIL function "@$s4test11FileStorageV8savePath_10storageKeyyx_SStAC9SaveErrorOyx_GYKF".
 for 'savePath(_:storageKey:)' (at /Users/lancelot/Downloads/test/Sources/main.swift:4:2)
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           0x00000001063c6a9c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x00000001063c4cf0 llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x00000001063c7068 SignalHandler(int) + 292
3  libsystem_platform.dylib 0x0000000191f1ee04 _sigtramp + 56
4  swift-frontend           0x00000001013ac4a8 (anonymous namespace)::IRGenSILFunction::emitSILFunction() + 6568
5  swift-frontend           0x00000001013ac4a8 (anonymous namespace)::IRGenSILFunction::emitSILFunction() + 6568
6  swift-frontend           0x00000001013aa65c swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 2088
7  swift-frontend           0x00000001011f5834 swift::irgen::IRGenerator::emitGlobalTopLevel(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>> const&) + 3364
8  swift-frontend           0x00000001013615a8 swift::IRGenRequest::evaluate(swift::Evaluator&, swift::IRGenDescriptor) const + 4700
9  swift-frontend           0x00000001013a9ba8 swift::SimpleRequest<swift::IRGenRequest, swift::GeneratedModule (swift::IRGenDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::IRGenRequest const&, swift::Evaluator&) + 176
10 swift-frontend           0x000000010136a394 swift::IRGenRequest::OutputType swift::Evaluator::getResultUncached<swift::IRGenRequest, swift::IRGenRequest::OutputType swift::evaluateOrFatal<swift::IRGenRequest>(swift::Evaluator&, swift::IRGenRequest)::'lambda'()>(swift::IRGenRequest const&, swift::IRGenRequest::OutputType swift::evaluateOrFatal<swift::IRGenRequest>(swift::Evaluator&, swift::IRGenRequest)::'lambda'()) + 784
11 swift-frontend           0x0000000101363e90 swift::performIRGeneration(swift::FileUnit*, swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::StringRef, llvm::GlobalVariable**) + 180
12 swift-frontend           0x0000000100d734bc generateIR(swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, swift::PrimarySpecificPaths const&, llvm::StringRef, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, llvm::GlobalVariable*&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>) + 156
13 swift-frontend           0x0000000100d6ece4 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*) + 2176
14 swift-frontend           0x0000000100d6dbd0 swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 984
15 swift-frontend           0x0000000100d70e88 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 1684
16 swift-frontend           0x0000000100d6fbb4 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3572
17 swift-frontend           0x0000000100cf6a5c swift::mainEntry(int, char const**) + 3680
18 dyld                     0x0000000191b68274 start + 2840

[giginet]

It is already fixed on the current trunk. (6b2fb2e)

https://swiftfiddle.com/dqntwdewobbcxlzksnermyaqae