Parameter pack compilation crash "This isn't an lvalue" with 6.0.3 OSS toolchain but works with Xcode toolchain

[marcprux]

Description

When attempting to build the parameter pack code in Reproduction, the Swift 6.0.3 OSS toolchain (~/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/bin/swift build) crashes, but the Swift 6.0.3 toolchain bundled with Xcode 16.2 (/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift build) builds fine.

Likely related to #78693 and #78671.

This was reported on our Slack (https://skiptools.slack.com/archives/C07MLMD57ST/p1741494643312329) when building for Android, which uses the OSS host toolchain.

Reproduction

@available(macOS 14, *)
public struct TupleComponent<each T: Component>: Component {
    public func write(_ data: (repeat (each T).Content)) { }
}

public protocol Component<Content> {
    associatedtype Content 
    func write(_ data: Content)
}

Stack dump

error: emit-module command failed due to signal 6 (use -v to see invocation)
Assertion failed: (isLValue() && "This isn't an lvalue"), function getLValueAddress, file ManagedValue.h, line 318.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
0.	Program arguments: /Users/marc/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/bin/swift-frontend -frontend -emit-module -experimental-skip-non-inlinable-function-bodies-without-types /private/tmp/TupleDemo/Sources/TupleDemo/TupleDemo.swift -target arm64-apple-macosx10.13 -Xllvm -aarch64-use-tbi -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -I /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/Modules -I /Users/marc/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/lib/swift/macosx/testing -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 /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/ModuleCache -swift-version 6 -Onone -D SWIFT_PACKAGE -D DEBUG -empty-abi-descriptor -plugin-path /Users/marc/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/lib/swift/host/plugins/testing -resource-dir /Users/marc/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/lib/swift -enable-anonymous-context-mangled-names -file-compilation-dir /private/tmp/TupleDemo -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 TupleDemo -package-name tupledemo -plugin-path /Users/marc/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Users/marc/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/local/lib/swift/host/plugins -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 -emit-module-doc-path /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/Modules/TupleDemo.swiftdoc -emit-module-source-info-path /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/Modules/TupleDemo.swiftsourceinfo -emit-objc-header-path /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/TupleDemo.build/TupleDemo-Swift.h -emit-dependencies-path /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/TupleDemo.build/TupleDemo.emit-module.d -parse-as-library -o /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/Modules/TupleDemo.swiftmodule -emit-abi-descriptor-path /private/tmp/TupleDemo/.build/arm64-apple-macosx/debug/Modules/TupleDemo.abi.json
1.	Apple Swift version 6.0.3 (swift-6.0.3-RELEASE)
2.	Compiling with the current language version
3.	While evaluating request ASTLoweringRequest(Lowering AST to SIL for module TupleDemo)
4.	While generating SIL witness table protocol conformance TupleComponent<repeat each T>: Component at 'TupleComponent' (at /private/tmp/TupleDemo/Sources/TupleDemo/TupleDemo.swift:5:8)
5.	While generating protocol witness thunk SIL function "@$s9TupleDemo0A9ComponentVyxxQp_QPGAA0C0A2aEP5writeyy7ContentQzFTW".
 for 'write(_:)' (at /private/tmp/TupleDemo/Sources/TupleDemo/TupleDemo.swift:6:12)
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           0x0000000107db80f8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x0000000107db68c4 llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x0000000107db8740 SignalHandler(int) + 304
3  libsystem_platform.dylib 0x000000018c18ede4 _sigtramp + 56
4  libsystem_pthread.dylib  0x000000018c157f70 pthread_kill + 288
5  libsystem_c.dylib        0x000000018c064908 abort + 128
6  libsystem_c.dylib        0x000000018c063c1c err + 0
7  swift-frontend           0x0000000107ef6244 (anonymous namespace)::TranslateArguments::expandSingleOuterInnerTuple(swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::TupleType>, swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::TupleType>, swift::Lowering::ManagedValue) (.cold.35) + 0
8  swift-frontend           0x0000000102fd4b28 (anonymous namespace)::TranslateArguments::expandSingleOuterInnerTuple(swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::TupleType>, swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::TupleType>, swift::Lowering::ManagedValue) + 1876
9  swift-frontend           0x0000000102fd2680 (anonymous namespace)::ExpanderBase<(anonymous namespace)::TranslateArguments, (anonymous namespace)::ParamInfo>::expand(swift::Lowering::AbstractionPattern, swift::CanType, swift::Lowering::AbstractionPattern, swift::CanType) + 616
10 swift-frontend           0x0000000102fd8c24 (anonymous namespace)::TranslateArguments::expandParam(swift::Lowering::AbstractionPattern, swift::AnyFunctionType::CanParam, swift::Lowering::AbstractionPattern, swift::AnyFunctionType::CanParam) + 224
11 swift-frontend           0x0000000102fcbc08 (anonymous namespace)::TranslateArguments::process(swift::Lowering::AbstractionPattern, swift::ArrayRefView<swift::AnyFunctionType::Param, swift::AnyFunctionType::CanParam, swift::AnyFunctionType::CanParam::getFromParam(swift::AnyFunctionType::Param const&), true>, swift::Lowering::AbstractionPattern, swift::ArrayRefView<swift::AnyFunctionType::Param, swift::AnyFunctionType::CanParam, swift::AnyFunctionType::CanParam::getFromParam(swift::AnyFunctionType::Param const&), true>, bool) + 2992
12 swift-frontend           0x0000000102fce160 swift::Lowering::SILGenFunction::emitProtocolWitness(swift::Lowering::AbstractionPattern, swift::CanTypeWrapper<swift::AnyFunctionType>, swift::SILDeclRef, swift::SubstitutionMap, swift::SILDeclRef, swift::SubstitutionMap, swift::Lowering::IsFreeFunctionWitness_t, bool, bool, std::__1::optional<swift::ActorIsolation>) + 2576
13 swift-frontend           0x0000000102ffa7c4 swift::Lowering::SILGenModule::emitProtocolWitness(swift::ProtocolConformanceRef, swift::SILLinkage, swift::SerializedKind_t, swift::SILDeclRef, swift::SILDeclRef, swift::Lowering::IsFreeFunctionWitness_t, swift::Witness) + 2992
14 swift-frontend           0x0000000102ffced0 (anonymous namespace)::SILGenWitnessTable<(anonymous namespace)::SILGenConformance>::addMethodImplementation(swift::SILDeclRef, swift::SILDeclRef, swift::Witness) + 528
15 swift-frontend           0x0000000102ffc848 (anonymous namespace)::SILGenWitnessTable<(anonymous namespace)::SILGenConformance>::addMethod(swift::SILDeclRef) + 532
16 swift-frontend           0x0000000102ffc398 swift::SILWitnessVisitor<(anonymous namespace)::SILGenConformance>::visitProtocolDecl(swift::ProtocolDecl*) + 1992
17 swift-frontend           0x0000000102ff9aa4 swift::Lowering::SILGenModule::getWitnessTable(swift::NormalProtocolConformance*) + 324
18 swift-frontend           0x0000000102ffb878 (anonymous namespace)::SILGenType::emitType() + 1580
19 swift-frontend           0x0000000102ffb240 swift::Lowering::SILGenModule::visitNominalTypeDecl(swift::NominalTypeDecl*) + 24
20 swift-frontend           0x0000000102eeb448 swift::ASTLoweringRequest::evaluate(swift::Evaluator&, swift::ASTLoweringDescriptor) const + 880
21 swift-frontend           0x0000000102fe90a4 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
22 swift-frontend           0x0000000102ef0d24 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
23 swift-frontend           0x0000000102eeba7c swift::performASTLowering(swift::ModuleDecl*, swift::Lowering::TypeConverter&, swift::SILOptions const&, swift::IRGenOptions const*) + 144
24 swift-frontend           0x00000001028ce358 swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 1176
25 swift-frontend           0x00000001028d9e5c withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 160
26 swift-frontend           0x00000001028d0028 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 708
27 swift-frontend           0x00000001028cf6fc swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2344
28 swift-frontend           0x00000001026e5edc swift::mainEntry(int, char const**) + 3096
29 dyld                     0x000000018bdd8274 start + 2840

Expected behavior

The OSS toolchain should compile the code without crashing just like the Xcode toolchain does.

Environment

zap /tmp/TupleDemo % ~/Library/Developer/Toolchains/swift-6.0.3-RELEASE.xctoolchain/usr/bin/swift -version
Apple Swift version 6.0.3 (swift-6.0.3-RELEASE)
Target: arm64-apple-macosx15.0

zap /tmp/TupleDemo % /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -version
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

zap /tmp/TupleDemo % sw_vers 
ProductName:		macOS
ProductVersion:		15.3.1
BuildVersion:		24D70

Xcode: Version 16.2 (16C5032a)

Additional information

No response