struct SetWrapper<T: Hashable> {
    let items: Set<T>
    init<S: Sequence>(events: S) where S.Iterator.Element == T {
        self.items = Set(events)
    }
}
extension SetWrapper: Collection, ExpressibleByArrayLiteral {
    typealias Index = Set<Element>.Index
    typealias Element = T
    
    init(arrayLiteral elements: Element...) {
        self = SetWrapper(events: elements)
    }
    var startIndex: Index {
        return items.startIndex
    }
    var endIndex: Index {
        return items.endIndex
    }
    func index(after index: Index) -> Index {
        return items.index(after: index)
    }
    subscript(index: Index) -> Element {
        return items[index]
    }
    func flatMap<O: Hashable>(_ transform: (Element) throws -> O?) rethrows -> SetWrapper<O> {
        return try SetWrapper(events: lazy.flatMap(transform))
    }
}

0  swift                    0x0000000104b98dba PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x0000000104b981f6 SignalHandler(int) + 662
2  libsystem_platform.dylib 0x00007fffb8b70b3a _sigtramp + 26
3  libsystem_platform.dylib 000000000000000000 _sigtramp + 1195963616
4  swift                    0x00000001026f6c15 swift::NominalTypeDecl::hasFixedLayout() const + 21
5  swift                    0x000000010228a931 (anonymous namespace)::LowerType::visitAnyStructType(swift::CanType, swift::StructDecl*) + 49
6  swift                    0x00000001022896c7 swift::Lowering::TypeConverter::getTypeLowering(swift::Lowering::AbstractionPattern, swift::Type) + 2791
7  swift                    0x0000000101d58df4 swift::Lowering::ResultPlanBuilder::build(swift::Lowering::Initialization*, swift::Lowering::AbstractionPattern, swift::CanType) + 692
8  swift                    0x0000000101d5ac4a swift::Lowering::ResultPlanBuilder::computeResultPlan(swift::Lowering::SILGenFunction&, swift::Lowering::CalleeTypeInfo const&, swift::SILLocation, swift::Lowering::SGFContext) + 826
9  swift                    0x0000000101d78c55 (anonymous namespace)::CallEmission::apply(swift::Lowering::SGFContext) + 1509
10 swift                    0x0000000101dd0909 swift::ASTVisitor<(anonymous namespace)::RValueEmitter, swift::Lowering::RValue, void, void, void, void, void, swift::Lowering::SGFContext>::visit(swift::Expr*, swift::Lowering::SGFContext) + 105
11 swift                    0x0000000101e43394 swift::Lowering::SILGenFunction::emitReturnExpr(swift::SILLocation, swift::Expr*) + 740
12 swift                    0x0000000101e3da7a swift::ASTVisitor<(anonymous namespace)::StmtEmitter, void, void, void, void, void, void>::visit(swift::Stmt*) + 202
13 swift                    0x0000000101e3e007 swift::ASTVisitor<(anonymous namespace)::StmtEmitter, void, void, void, void, void, void>::visit(swift::Stmt*) + 1623
14 swift                    0x0000000101df5389 swift::Lowering::SILGenFunction::emitFunction(swift::FuncDecl*) + 393
15 swift                    0x0000000101d6b3a1 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*)::$_1::operator()(swift::SILFunction*) const + 273
16 swift                    0x0000000101d6a919 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 761
17 swift                    0x0000000101e4d4c6 swift::Lowering::SILGenModule::visitExtensionDecl(swift::ExtensionDecl*) + 422
18 swift                    0x0000000101d70f6b swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*, unsigned int) + 1115
19 swift                    0x0000000101d728f9 swift::SILModule::constructSIL(swift::ModuleDecl*, swift::SILOptions&, swift::FileUnit*, llvm::Optional<unsigned int>, bool) + 841
20 swift                    0x000000010150b2c6 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 13014
21 swift                    0x0000000101506784 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7716
22 swift                    0x00000001014bb6a8 main + 12248
23 libdyld.dylib            0x00007fffb8961235 start + 1
Stack dump:
0.  Program arguments: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file test.swift -target x86_64-apple-macosx10.9 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -color-diagnostics -module-name test -o /var/folders/sg/7mgms6wn1zs529t4gqfljmh40000gp/T/test-70c920.o
1.  While emitting SIL for 'flatMap(_:)' at test.swift:26:5
<unknown>:0: error: unable to execute command: Segmentation fault: 11
<unknown>:0: error: compile command failed due to signal 11 (use -v to see invocation)