public protocol ResponseBodyStreamer { }
public enum ResponseBody {
    case stream(ResponseBodyStreamer)
    case empty
}

public final class Response {
    public var body: ResponseBody
    public init(body: ResponseBody) {
        self.body = body
    }
}

public func apply() async throws -> Response {
    return Response(body: .empty)
}

1.  Apple Swift version 5.4-dev (LLVM 1e4181b99f530d2, Swift 842cc9c88f5463b)
2.  While emitting IR SIL function "@$s11Hummingbird5applyAA8ResponseCyYKF".
 for 'apply()' (at /Users/adamfowler/Developer/server/hummingbird/hummingbird/Sources/Hummingbird/File.swift:16:8)
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           0x0000000113964857 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 39
1  swift-frontend           0x0000000113963a15 llvm::sys::RunSignalHandlers() + 85
2  swift-frontend           0x0000000113964e76 SignalHandler(int) + 262
3  libsystem_platform.dylib 0x00007fff20373d7d _sigtramp + 29
4  libsystem_platform.dylib 000000000000000000 _sigtramp + 18446603339975672480
5  libsystem_c.dylib        0x00007fff20282720 abort + 120
6  libsystem_c.dylib        0x00007fff202819d6 err + 0
7  swift-frontend           0x0000000113a810b3 swift::SILInstructionVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::SILInstruction*) (.cold.744) + 35
8  swift-frontend           0x000000010f33d737 swift::SILInstructionVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::SILInstruction*) + 76631
9  swift-frontend           0x000000010f32621f (anonymous namespace)::IRGenSILFunction::emitSILFunction() + 8063
10 swift-frontend           0x000000010f323d36 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 1542
11 swift-frontend           0x000000010f1f5c32 swift::irgen::IRGenerator::emitGlobalTopLevel(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) + 818
12 swift-frontend           0x000000010f2e86f7 swift::performIRGeneration(swift::ModuleDecl*, swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, llvm::GlobalVariable**) + 1431
13 swift-frontend           0x000000010effbb62 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*) + 2386
14 swift-frontend           0x000000010effafec performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 636
15 swift-frontend           0x000000010eff0028 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 5016
16 swift-frontend           0x000000010ef8a566 main + 566
17 libdyld.dylib            0x00007fff2034a621 start + 1