func test(with strings: [String], includeAnything: Bool = true) -> String {
    return strings.joined(separator: ",")
}

func test(with strings: String..., includeAnything: Bool = true) -> String {
    return test(with: strings, includeAnything: includeAnything)
}

$ swiftc EmitDefaultArgGeneratorCrash.swift
0  swift                    0x0000000111cb27a7 PrintStackTraceSignalHandler(void*) + 39
1  swift                    0x0000000111cb1d36 SignalHandler(int) + 662
2  libsystem_platform.dylib 0x00007fffca4e0bba _sigtramp + 26
3  libsystem_platform.dylib 0x00007fad4e8e9000 _sigtramp + 2218820704
4  swift                    0x000000010f6056ee swift::Lowering::SILGenModule::emitAbstractFuncDecl(swift::AbstractFunctionDecl*) + 1086
5  swift                    0x000000010f604e9f swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 31
6  swift                    0x000000010f613a81 swift::ASTVisitor<swift::Lowering::SILGenModule, void, void, void, void, void, void>::visit(swift::Decl*) + 657
7  swift                    0x000000010f612b3b swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*, unsigned int) + 1483
8  swift                    0x000000010f614719 swift::SILModule::constructSIL(swift::ModuleDecl*, swift::SILOptions&, swift::FileUnit*, llvm::Optional<unsigned int>, bool, bool) + 1593
9  swift                    0x000000010ef393f2 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 45842
10 swift                    0x000000010eee93c2 main + 11298
11 libdyld.dylib            0x00007fffca2d3255 start + 1
12 libdyld.dylib            0x000000000000000f start + 903007675
Stack dump:
0.  Program arguments: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file EmitDefaultArgGeneratorCrash.swift -target x86_64-apple-macosx10.9 -enable-objc-interop -sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -color-diagnostics -module-name EmitDefaultArgGeneratorCrash -o /var/folders/0z/101fmx1n5hl3g6l0vrn5g9qh0000gn/T/EmitDefaultArgGeneratorCrash-2ecac5.o 
1.  While silgen emitDefaultArgGenerator  SIL function @_TIF28EmitDefaultArgGeneratorCrash4testFT4withGSaSS_15includeAnythingSb_SSA0_ for expression at [EmitDefaultArgGeneratorCrash.swift:5:60 - line:5:60] RangeText="t"
<unknown>:0: error: unable to execute command: Segmentation fault: 11
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)