protocol BarProtocol {}

extension BarProtocol {
    var str: String { return "str" }
}

struct Foo<T: AnyObject>: BarProtocol {
    unowned let bar: T
}

class Bar {
    let str2 = "str2"

    var foo: BarProtocol {
        return Foo(bar: self)
    }
}

_ = Bar().foo.str

0  swift                    0x000000010f514efa PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x000000010f5146dc SignalHandler(int) + 348
2  libsystem_platform.dylib 0x00007fff7c49cb3d _sigtramp + 29
3  libsystem_platform.dylib 0x0000000000000001 _sigtramp + 2209756385
4  libobjc.A.dylib          0x00007fff7b1dcfaa objc_copyWeak + 18
5  libswiftCore.dylib       0x000000011202be3c swift_unknownObjectUnownedCopyInit + 76
6  libswiftCore.dylib       0x0000000110c94365 swift_unknownObjectUnownedCopyInit + 4274423157
7  libswiftCore.dylib       0x0000000110c940a3 swift_unknownObjectUnownedCopyInit + 4274422451
8  swift                    0x000000010bd9d13d llvm::MCJIT::runFunction(llvm::Function*, llvm::ArrayRef<llvm::GenericValue>) + 365
9  swift                    0x000000010bda37fc llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*, 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&, char const* const*) + 1004
10 swift                    0x000000010b3867b1 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 54977
11 swift                    0x000000010b375ac7 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6823
12 swift                    0x000000010b31590d main + 1261
13 libdyld.dylib            0x00007fff7c2b1ed9 start + 1