This complicated combination of trailing closures and autoclosures crashes the Swift compiler

[schuelermine]

Description

No response

Reproduction

func foo(x: @autoclosure () -> () = (), _ y: @autoclosure () -> (), z: @autoclosure () -> (), w: () -> ()) { }

foo(x: ()) { } z: { } w: { }

Stack dump

Planning build
Building for debugging...
Write auxiliary file /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift-version-575752FA6EFF9271.txt
/opt/swift-5.10.1/bin/swiftc -module-name swift_crash -emit-dependencies -emit-module -emit-module-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.swiftmodule -output-file-map /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/output-file-map.json -incremental -c @/home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/sources -I /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug -target x86_64-unknown-linux-gnu -swift-version 5 -v -enable-batch-mode -index-store-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/index/store -Onone -enable-testing -j16 -DSWIFT_PACKAGE -DDEBUG -module-cache-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/ModuleCache -parseable-output -Xfrontend -entry-point-function-name -Xfrontend swift_crash_main -g -Xcc -fPIC -Xcc -g -package-name swift_crash -Xcc -fno-omit-frame-pointer
Swift version 5.10.1 (swift-5.10.1-RELEASE)
Target: x86_64-unknown-linux-gnu
/opt/swift-5.10.1/bin/swift-frontend -frontend -emit-module -experimental-skip-non-inlinable-function-bodies-without-types /home/ubuntu/Documents/swift-crash/Sources/main.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -I /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug -enable-testing -g -module-cache-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/ModuleCache -swift-version 5 -Onone -D SWIFT_PACKAGE -D DEBUG -new-driver-path /opt/swift-5.10.1/bin/swift-driver -entry-point-function-name swift_crash_main -empty-abi-descriptor -resource-dir /opt/swift-5.10.1/lib/swift -enable-anonymous-context-mangled-names -Xcc -fPIC -Xcc -g -Xcc -fno-omit-frame-pointer -module-name swift_crash -package-name swift_crash -plugin-path /opt/swift-5.10.1/lib/swift/host/plugins -plugin-path /opt/swift-5.10.1/local/lib/swift/host/plugins -emit-module-doc-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.swiftdoc -emit-module-source-info-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.swiftsourceinfo -emit-dependencies-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/swift_crash.emit-module.d -o /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.swiftmodule
/opt/swift-5.10.1/bin/swift-frontend -frontend -c -primary-file /home/ubuntu/Documents/swift-crash/Sources/main.swift -emit-dependencies-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/main.d -emit-reference-dependencies-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/main.swiftdeps -target x86_64-unknown-linux-gnu -disable-objc-interop -I /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug -enable-testing -g -module-cache-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/ModuleCache -swift-version 5 -Onone -D SWIFT_PACKAGE -D DEBUG -new-driver-path /opt/swift-5.10.1/bin/swift-driver -entry-point-function-name swift_crash_main -empty-abi-descriptor -resource-dir /opt/swift-5.10.1/lib/swift -enable-anonymous-context-mangled-names -Xcc -fPIC -Xcc -g -Xcc -fno-omit-frame-pointer -module-name swift_crash -package-name swift_crash -plugin-path /opt/swift-5.10.1/lib/swift/host/plugins -plugin-path /opt/swift-5.10.1/local/lib/swift/host/plugins -o /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/main.swift.o -index-store-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/index/store -index-system-modules
error: compile command failed due to signal 6 (use -v to see invocation)
swift-frontend: /home/build-user/swift/lib/Sema/MiscDiagnostics.cpp:2154: bool swift::diagnoseArgumentLabelError(ASTContext &, const ArgumentList *, ArrayRef<Identifier>, ParameterContext, InFlightDiagnostic *): Assertion `numMissing > 0 || numExtra > 0 || numWrong > 0' failed.
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
0.	Program arguments: /opt/swift-5.10.1/bin/swift-frontend -frontend -c -primary-file /home/ubuntu/Documents/swift-crash/Sources/main.swift -emit-dependencies-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/main.d -emit-reference-dependencies-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/main.swiftdeps -target x86_64-unknown-linux-gnu -disable-objc-interop -I /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug -enable-testing -g -module-cache-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/ModuleCache -swift-version 5 -Onone -D SWIFT_PACKAGE -D DEBUG -new-driver-path /opt/swift-5.10.1/bin/swift-driver -entry-point-function-name swift_crash_main -empty-abi-descriptor -resource-dir /opt/swift-5.10.1/lib/swift -enable-anonymous-context-mangled-names -Xcc -fPIC -Xcc -g -Xcc -fno-omit-frame-pointer -module-name swift_crash -package-name swift_crash -plugin-path /opt/swift-5.10.1/lib/swift/host/plugins -plugin-path /opt/swift-5.10.1/local/lib/swift/host/plugins -o /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/swift_crash.build/main.swift.o -index-store-path /home/ubuntu/Documents/swift-crash/.build/x86_64-unknown-linux-gnu/debug/index/store -index-system-modules
1.	Swift version 5.10.1 (swift-5.10.1-RELEASE)
2.	Compiling with the current language version
3.	While evaluating request TypeCheckSourceFileRequest(source_file "/home/ubuntu/Documents/swift-crash/Sources/main.swift")
4.	While type-checking statement at [/home/ubuntu/Documents/swift-crash/Sources/main.swift:3:1 - line:3:28] RangeText="foo(x: ()) { } z: { } w: { "
5.	While type-checking expression at [/home/ubuntu/Documents/swift-crash/Sources/main.swift:3:1 - line:3:28] RangeText="foo(x: ()) { } z: { } w: { "
6.	While type-checking-target starting at /home/ubuntu/Documents/swift-crash/Sources/main.swift:3:1
 #0 0x00005640c43dffe4 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/swift-5.10.1/bin/swift-frontend+0x72effe4)
 #1 0x00005640c43ddd4e llvm::sys::RunSignalHandlers() (/opt/swift-5.10.1/bin/swift-frontend+0x72edd4e)
 #2 0x00005640c43e0358 SignalHandler(int) Signals.cpp:0:0
 #3 0x00007f371e245320 (/lib/x86_64-linux-gnu/libc.so.6+0x45320)
 #4 0x00007f371e29eb1c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb1c)
 #5 0x00007f371e24526e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4526e)
 #6 0x00007f371e2288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
 #7 0x00007f371e22881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
 #8 0x00007f371e23b507 (/lib/x86_64-linux-gnu/libc.so.6+0x3b507)
 #9 0x00005640bf27df8d swift::diagnoseArgumentLabelError(swift::ASTContext&, swift::ArgumentList const*, llvm::ArrayRef<swift::Identifier>, swift::ParameterContext, swift::InFlightDiagnostic*) (/opt/swift-5.10.1/bin/swift-frontend+0x218df8d)
#10 0x00005640bf1d16cc swift::constraints::RelabelArguments::diagnose(swift::constraints::Solution const&, bool) const (/opt/swift-5.10.1/bin/swift-frontend+0x20e16cc)
#11 0x00005640bf48bd1f swift::constraints::ConstraintSystem::applySolutionFixes(swift::constraints::Solution const&) (/opt/swift-5.10.1/bin/swift-frontend+0x239bd1f)
#12 0x00005640bf48c5c9 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::constraints::SyntacticElementTarget) (/opt/swift-5.10.1/bin/swift-frontend+0x239c5c9)
#13 0x00005640bf31c738 swift::TypeChecker::typeCheckTarget(swift::constraints::SyntacticElementTarget&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) (/opt/swift-5.10.1/bin/swift-frontend+0x222c738)
#14 0x00005640bf31c56d swift::TypeChecker::typeCheckExpression(swift::constraints::SyntacticElementTarget&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) (/opt/swift-5.10.1/bin/swift-frontend+0x222c56d)
#15 0x00005640bf31c454 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::constraints::ContextualTypeInfo, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) (/opt/swift-5.10.1/bin/swift-frontend+0x222c454)
#16 0x00005640bf42c143 (anonymous namespace)::StmtChecker::typeCheckASTNode(swift::ASTNode&) TypeCheckStmt.cpp:0:0
#17 0x00005640bf42fc2d swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) TypeCheckStmt.cpp:0:0
#18 0x00005640bf42dc6c bool (anonymous namespace)::StmtChecker::typeCheckStmt<swift::BraceStmt>(swift::BraceStmt*&) TypeCheckStmt.cpp:0:0
#19 0x00005640bf42dd01 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) (/opt/swift-5.10.1/bin/swift-frontend+0x233dd01)
#20 0x00005640bf470062 swift::TypeCheckSourceFileRequest::evaluate(swift::Evaluator&, swift::SourceFile*) const (/opt/swift-5.10.1/bin/swift-frontend+0x2380062)
#21 0x00005640bf472734 llvm::Expected<swift::TypeCheckSourceFileRequest::OutputType> swift::Evaluator::getResultUncached<swift::TypeCheckSourceFileRequest>(swift::TypeCheckSourceFileRequest const&) crtstuff.c:0:0
#22 0x00005640bf4725be llvm::Expected<swift::TypeCheckSourceFileRequest::OutputType> swift::Evaluator::getResultCached<swift::TypeCheckSourceFileRequest, (void*)0>(swift::TypeCheckSourceFileRequest const&) crtstuff.c:0:0
#23 0x00005640bf46fea4 swift::TypeCheckSourceFileRequest::OutputType swift::evaluateOrDefault<swift::TypeCheckSourceFileRequest>(swift::Evaluator&, swift::TypeCheckSourceFileRequest, swift::TypeCheckSourceFileRequest::OutputType) (/opt/swift-5.10.1/bin/swift-frontend+0x237fea4)
#24 0x00005640bf46fe84 swift::performTypeChecking(swift::SourceFile&) (/opt/swift-5.10.1/bin/swift-frontend+0x237fe84)
#25 0x00005640be1c9909 bool llvm::function_ref<bool (swift::SourceFile&)>::callback_fn<swift::CompilerInstance::performSema()::$_7>(long, swift::SourceFile&) Frontend.cpp:0:0
#26 0x00005640be1be43a swift::CompilerInstance::forEachFileToTypeCheck(llvm::function_ref<bool (swift::SourceFile&)>) (/opt/swift-5.10.1/bin/swift-frontend+0x10ce43a)
#27 0x00005640be1be39d swift::CompilerInstance::performSema() (/opt/swift-5.10.1/bin/swift-frontend+0x10ce39d)
#28 0x00005640bdf1ed12 withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) FrontendTool.cpp:0:0
#29 0x00005640bdf0c51b performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) FrontendTool.cpp:0:0
#30 0x00005640bdf0af2f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/opt/swift-5.10.1/bin/swift-frontend+0xe1af2f)
#31 0x00005640bddb0872 swift::mainEntry(int, char const**) (/opt/swift-5.10.1/bin/swift-frontend+0xcc0872)
#32 0x00007f371e22a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#33 0x00007f371e22a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#34 0x00005640bddaf855 _start (/opt/swift-5.10.1/bin/swift-frontend+0xcbf855)

Expected behavior

I don’t actually know if this program should be accepted or not.

Environment

Swift version 5.10.1 (swift-5.10.1-RELEASE)
Target: x86_64-unknown-linux-gnu

Additional information

No response

[xwu]

This is an interesting case. Indeed, the code is invalid, but the compiler shouldn't crash trying to diagnose it.

In the REPL on macOS, there is no crash, but I can indeed reproduce this on godbolt.org with both the nightly and Swift 5.10 compiler. It appears to be a relatively recent regression as Swift 5.9 doesn't crash but rather correctly diagnoses the problem(s).

[schuelermine]

Interesting. Entering the first line alone into the REPL crashes the REPL on Ubuntu.

[schuelermine]

ubuntu@swift:~$ swift repl -v
Swift version 5.10.1 (swift-5.10.1-RELEASE)
Target: x86_64-unknown-linux-gnu
/opt/swift-5.10.1/bin/lldb '--repl=-disable-objc-interop -color-diagnostics -new-driver-path /opt/swift-5.10.1/bin/swift-driver -empty-abi-descriptor -resource-dir /opt/swift-5.10.1/lib/swift -plugin-path /opt/swift-5.10.1/lib/swift/host/plugins -plugin-path /opt/swift-5.10.1/local/lib/swift/host/plugins'
Welcome to Swift version 5.10.1 (swift-5.10.1-RELEASE).
Type :help for assistance.
  1> func foo(x: @autoclosure () -> () = (), _ y: @autoclosure () -> (), z: @autoclosure () -> (), w: () -> ()) { }
SIL verification failed: function_ref inside fragile function cannot reference a private or hidden symbol: (SingleFunction && RefF->isExternalDeclaration()) || RefF->hasValidLinkageForFragileRef()
Verifying instruction:
->   // function_ref implicit closure #1 in default argument 0 of foo(x:_:z:w:)
  %0 = function_ref @$s13__lldb_expr_13foo1x_1z1wyyyXK_yyXKyyXKyyXEtFfA_yycfu_ : $@convention(thin) () -> () // user: %1
     %1 = thin_to_thick_function %0 : $@convention(thin) () -> () to $@callee_guaranteed () -> () // user: %2
In function:
// default argument 0 of foo(x:_:z:w:)
sil non_abi [serialized] [ossa] @$s13__lldb_expr_13foo1x_1z1wyyyXK_yyXKyyXKyyXEtFfA_ : $@convention(thin) () -> @owned @callee_guaranteed () -> () {
bb0:
  // function_ref implicit closure #1 in default argument 0 of foo(x:_:z:w:)
  %0 = function_ref @$s13__lldb_expr_13foo1x_1z1wyyyXK_yyXKyyXKyyXEtFfA_yycfu_ : $@convention(thin) () -> () // user: %1
  %1 = thin_to_thick_function %0 : $@convention(thin) () -> () to $@callee_guaranteed () -> () // user: %2
  return %1 : $@callee_guaranteed () -> ()        // id: %2
} // end sil function '$s13__lldb_expr_13foo1x_1z1wyyyXK_yyXKyyXKyyXEtFfA_'

PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
 #0 0x0000564c84634ec4 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/swift-5.10.1/bin/lldb+0x28ec4)
 #1 0x0000564c8463301e llvm::sys::RunSignalHandlers() (/opt/swift-5.10.1/bin/lldb+0x2701e)
 #2 0x0000564c846354c8 SignalHandler(int) Signals.cpp:0:0
 #3 0x00007f8478245320 (/lib/x86_64-linux-gnu/libc.so.6+0x45320)
 #4 0x00007f847829eb1c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb1c)
 #5 0x00007f847824526e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4526e)
 #6 0x00007f84782288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
 #7 0x00007f847b24bc8e (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x284bc8e)
 #8 0x00007f847b272554 (anonymous namespace)::SILVerifier::checkFunctionRefBaseInst(swift::FunctionRefBaseInst*) SILVerifier.cpp:0:0
 #9 0x00007f847b250bd4 swift::SILVisitorBase<(anonymous namespace)::SILVerifier, void>::visitSILBasicBlock(swift::SILBasicBlock*) SILVerifier.cpp:0:0
#10 0x00007f847b2504af (anonymous namespace)::SILVerifier::visitSILBasicBlock(swift::SILBasicBlock*) SILVerifier.cpp:0:0
#11 0x00007f847b24eb2e (anonymous namespace)::SILVerifier::visitSILFunction(swift::SILFunction*) SILVerifier.cpp:0:0
#12 0x00007f847b247599 swift::SILFunction::verify(swift::SILPassManager*, bool, bool, bool) const (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x2847599)
#13 0x00007f847af833fe swift::Lowering::SILGenModule::postEmitFunction(swift::SILDeclRef, swift::SILFunction*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x25833fe)
#14 0x00007f847af82650 swift::Lowering::SILGenModule::emitFunctionDefinition(swift::SILDeclRef, swift::SILFunction*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x2582650)
#15 0x00007f847af83c6f swift::Lowering::SILGenModule::emitOrDelayFunction(swift::SILDeclRef) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x2583c6f)
#16 0x00007f847af84bfe swift::Lowering::SILGenModule::emitArgumentGenerators(llvm::PointerUnion<swift::ValueDecl*, swift::AbstractClosureExpr*, swift::FileUnit*>, swift::ParameterList*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x2584bfe)
#17 0x00007f847af847fc swift::Lowering::SILGenModule::emitAbstractFuncDecl(swift::AbstractFunctionDecl*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x25847fc)
#18 0x00007f847af81242 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x2581242)
#19 0x00007f847af86d67 swift::ASTLoweringRequest::evaluate(swift::Evaluator&, swift::ASTLoweringDescriptor) const (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x2586d67)
#20 0x00007f847b05ee0b swift::SimpleRequest<swift::ASTLoweringRequest, std::unique_ptr<swift::SILModule, std::default_delete<swift::SILModule>> (swift::ASTLoweringDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::ASTLoweringRequest const&, swift::Evaluator&) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x265ee0b)
#21 0x00007f847aa0adb3 llvm::Expected<swift::ASTLoweringRequest::OutputType> swift::Evaluator::getResultUncached<swift::ASTLoweringRequest>(swift::ASTLoweringRequest const&) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x200adb3)
#22 0x00007f847af878c8 swift::performASTLowering(swift::FileUnit&, swift::Lowering::TypeConverter&, swift::SILOptions const&, swift::IRGenOptions const*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x25878c8)
#23 0x00007f847a242de3 lldb_private::SwiftExpressionParser::Parse(lldb_private::DiagnosticManager&, unsigned int, unsigned int) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x1842de3)
#24 0x00007f847a23a4c5 lldb_private::SwiftUserExpression::GetTextAndSetExpressionParser(lldb_private::DiagnosticManager&, std::unique_ptr<lldb_private::SwiftExpressionSourceCode, std::default_delete<lldb_private::SwiftExpressionSourceCode>>&, lldb_private::ExecutionContext&, lldb_private::ExecutionContextScope*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x183a4c5)
#25 0x00007f847a23b507 lldb_private::SwiftUserExpression::Parse(lldb_private::DiagnosticManager&, lldb_private::ExecutionContext&, lldb_private::ExecutionPolicy, bool, bool) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x183b507)
#26 0x00007f8479a802f2 lldb_private::UserExpression::Evaluate(lldb_private::ExecutionContext&, lldb_private::EvaluateExpressionOptions const&, llvm::StringRef, llvm::StringRef, std::shared_ptr<lldb_private::ValueObject>&, lldb_private::Status&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, lldb_private::ValueObject*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x10802f2)
#27 0x00007f8479a7c9d2 lldb_private::REPL::IOHandlerInputComplete(lldb_private::IOHandler&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>&) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x107c9d2)
#28 0x00007f8479a039f9 lldb_private::IOHandlerEditline::Run() (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x10039f9)
#29 0x00007f84799e23a6 lldb_private::Debugger::RunIOHandlers() (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0xfe23a6)
#30 0x00007f84799ea342 std::_Function_handler<void* (), lldb_private::Debugger::StartIOHandlerThread()::$_4>::_M_invoke(std::_Any_data const&) Debugger.cpp:0:0
#31 0x00007f8479aae060 lldb_private::HostNativeThreadBase::ThreadCreateTrampoline(void*) (/opt/swift-5.10.1/bin/../lib/liblldb.so.15git+0x10ae060)
#32 0x00007f847829ca94 (/lib/x86_64-linux-gnu/libc.so.6+0x9ca94)
#33 0x00007f8478329c3c (/lib/x86_64-linux-gnu/libc.so.6+0x129c3c)
Aborted (core dumped)

[hamishknight]

Looks like you don't even need the autoclosure to reproduce, this also crashes:

func foo(_ y: Int, w: () -> ()) { }
foo { } w: { }