import Cocoa

struct InnerA {
    
}

struct InnerB {
    
}

enum Storage {
    case innerA(InnerA)
    case innerB(InnerB)
}

struct Outer {
    var storage: Storage?
    public var innerA: InnerA {
        get {
            if case .innerA(let v)? = storage { return v }
            return InnerA()
        }
        set { storage = .innerA(newValue) }
    }
    public var innerB: InnerB {
        get {
            if case .innerB(let v)? = storage { return v }
            return InnerB()
        }
        set { storage = .innerB(newValue) }
    }
    init() {
        storage = nil
    }
}

class Foo<InnerType> {
    let somePath: WritableKeyPath<Outer, InnerType>!
    init(_ path: WritableKeyPath<Outer, InnerType>, inner: InnerType) {
        self.somePath = path
        self.innerInstance = inner
        self.outerInstance = Outer()
    }
    
    var innerInstance: InnerType
    var outerInstance: Outer
    
    func update() {
        outerInstance[keyPath: somePath] = innerInstance
    }
}

0  swift                    0x0000000111fe658a PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x0000000111fe59c6 SignalHandler(int) + 662
2  libsystem_platform.dylib 0x00007fff77e1bf5a _sigtramp + 26
3  swift                    0x000000010fb6a2c6 bool llvm::function_ref<bool (swift::Type, llvm::ArrayRef<swift::Requirement>)>::callback_fn<swift::GenericSignature::getSubstitutions(swift::SubstitutionMap const&, llvm::SmallVectorImpl<swift::Substitution>&) const::$_8>(long, swift::Type, llvm::ArrayRef<swift::Requirement>) + 150
4  swift                    0x000000010f8ef58f (anonymous namespace)::ExprRewriter::buildSubscript(swift::Expr*, swift::Expr*, llvm::ArrayRef<swift::Identifier>, bool, swift::constraints::ConstraintLocatorBuilder, bool, swift::AccessSemantics) + 1791
5  swift                    0x000000010f8de0bf swift::ASTVisitor<(anonymous namespace)::ExprRewriter, swift::Expr*, void, void, void, void, void>::visit(swift::Expr*) + 2799
6  swift                    0x000000010f8dd309 (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) + 25
7  swift                    0x000000010fb21efa swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Expr*) + 11354
8  swift                    0x000000010f9faa11 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) + 1537
9  swift                    0x000000010fa8042d swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 13869
10 swift                    0x000000010fa7c2b2 swift::TypeChecker::typeCheckAbstractFunctionBodyUntil(swift::AbstractFunctionDecl*, swift::SourceLoc) + 1090
11 swift                    0x000000010fa81c6b swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 475
12 swift                    0x000000010fa9f86a swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int) + 2506
13 swift                    0x000000010f5c9427 swift::CompilerInstance::performSema() + 5031
14 swift                    0x000000010e93fcfb performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1371
15 swift                    0x000000010e93df34 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7716
16 swift                    0x000000010e8f2bb8 main + 12248
17 libdyld.dylib            0x00007fff77b9b145 start + 1
18 libdyld.dylib            0x0000000000000044 start + 2286309120