protocol C : class, AnyObject {}

Assertion failed: (!ty->hasError() && "Serializing error type"), function addTypeRef, file /Users/Hamish/Desktop/swift-dev/swift/lib/Serialization/Serialization.cpp, line 625.
0  swift                    0x0000000105ba18f8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x0000000105ba08f7 llvm::sys::RunSignalHandlers() + 39
2  swift                    0x0000000105ba1f72 SignalHandler(int) + 258
3  libsystem_platform.dylib 0x00007fff58223f5a _sigtramp + 26
4  swift                    0x0000000106f3fdc0 cmark_strbuf__initbuf + 114443
5  libsystem_c.dylib        0x00007fff57fc11ae abort + 127
6  libsystem_c.dylib        0x00007fff57f891ac basename_r + 0
7  swift                    0x00000001031727a0 swift::serialization::Serializer::addTypeRef(swift::Type) + 416
8  swift                    0x00000001031826ec swift::serialization::Serializer::writeDecl(swift::Decl const*) + 10908
9  swift                    0x000000010318b642 swift::serialization::Serializer::writeAllDeclsAndTypes() + 3490
10 swift                    0x000000010318cb73 swift::serialization::Serializer::writeAST(llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, bool) + 3603
11 swift                    0x0000000103193eeb swift::serialization::Serializer::writeToStream(llvm::raw_ostream&, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::SILModule const*, swift::SerializationOptions const&) + 139
12 swift                    0x00000001031d8c03 void llvm::function_ref<void (llvm::raw_ostream&)>::callback_fn<swift::serialize(llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::SerializationOptions const&, swift::SILModule const*)::$_3>(long, llvm::raw_ostream&) + 179
13 swift                    0x0000000103195148 withOutputFile(swift::ASTContext&, llvm::StringRef, llvm::function_ref<void (llvm::raw_ostream&)>) + 440
14 swift                    0x0000000103194f0c swift::serialize(llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::SerializationOptions const&, swift::SILModule const*) + 220
15 swift                    0x00000001021881d4 std::__1::__function::__func<performCompileStepsPostSILGen(swift::CompilerInstance&, swift::CompilerInvocation&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, bool, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, bool, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*)::$_10, std::__1::allocator<performCompileStepsPostSILGen(swift::CompilerInstance&, swift::CompilerInvocation&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, bool, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, bool, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*)::$_10>, void ()>::operator()() + 500
16 swift                    0x0000000102ec1de4 swift::SILModule::serialize() + 36
17 swift                    0x000000010217e5ec performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 14892
18 swift                    0x0000000102179b5a swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3274
19 swift                    0x0000000102134588 main + 1128
20 libdyld.dylib            0x00007fff57f15015 start + 1
Stack dump:
0.  Program arguments: ./swift -frontend -c -primary-file /Users/Hamish/Desktop/main.swift -emit-module-path /dev/null -o /dev/null
1.  While serializing 'C' at /Users/Hamish/Desktop/main.swift:11:1
2.  While serializing type '<<error type>>'
fish: './swift -frontend -c -primary-f…' terminated by signal SIGABRT (Abort)