$ cat ProblematicCode.swift
func outerFunc(outerParam: Int) {
   func innerFunc(innerParam: Int = outerParam) {}
   innerFunc()
}
outerFunc(outerParam: 2)

$ swift ProblematicCode.swift
Global is external, but doesn't have external or weak linkage!
i8* ()* @_T015ProblematicCode9outerFuncySi0C5Param_tFACL_Sivau
<unknown>:0: error: fatal error encountered during compilation; please file a bug report with your project and the crash log
<unknown>:0: note: Broken module found, compilation aborted!
0 swift 0x0000000107863ffa PrintStackTraceSignalHandler(void*) + 42
1 swift 0x00000001078633b6 SignalHandler(int) + 966
2 libsystem_platform.dylib 0x00007fff61aa8f5a _sigtramp + 26
3 libsystem_platform.dylib 0x00007ffeebdb83a8 _sigtramp + 2318464104
4 libsystem_c.dylib 0x00007fff618461ae abort + 127
5 swift 0x0000000103eba8e7 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*)::$_0::__invoke(void*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool) + 519
6 swift 0x000000010781b658 llvm::report_fatal_error(llvm::Twine const&, bool) + 280
7 swift 0x000000010781b539 llvm::report_fatal_error(char const*, bool) + 41
8 swift 0x00000001077e3c5c (anonymous namespace)::VerifierLegacyPass::doFinalization(llvm::Module&) + 204
9 swift 0x000000010778b015 llvm::FPPassManager::doFinalization(llvm::Module&) + 53
10 swift 0x0000000107790167 llvm::legacy::FunctionPassManagerImpl::doFinalization(llvm::Module&) + 71
11 swift 0x0000000104063a35 swift::performLLVM(swift::IRGenOptions&, swift::DiagnosticEngine*, llvm::sys::SmartMutex<false>*, llvm::GlobalVariable*, llvm::Module*, llvm::TargetMachine*, swift::version::Version const&, llvm::StringRef, swift::UnifiedStatsReporter*) + 5125
12 swift 0x0000000104068a62 performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, llvm::LLVMContext&, swift::SourceFile*, llvm::GlobalVariable**, unsigned int) + 2738
13 swift 0x0000000104065a52 swift::performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, llvm::LLVMContext&, llvm::GlobalVariable**) + 1746
14 swift 0x0000000103ec536b performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 43643
15 swift 0x0000000103eb8e64 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7908
16 swift 0x0000000103e6d8b5 main + 18917
17 libdyld.dylib 0x00007fff6179a015 start + 1
18 libdyld.dylib 0x000000000000000a start + 2659606518
Stack dump:
0. Program arguments: /Users/felixscheinost/Downloads/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -interpret ProblematicCode.swift -enable-objc-interop -sdk /Users/felixscheinost/Downloads/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -color-diagnostics -module-name ProblematicCode
fish: 'swift ProblematicCode.swift' terminated by signal SIGABRT (Abort)