Open Radar

 
Summary:
Having removed the use of Box from a single-payload enum, this branch of my framework Traversal deadlocks when you run the tests: https://github.com/robrix/Traversal/pull/19


Steps to Reproduce:
1. Clone & check out the branch in this PR: https://github.com/robrix/Traversal/pull/19
2. Open Traversal.xcworkspace
3. Select the scheme for Traversal if it isn’t already selected
4. Run the tests


Expected Results:
I expected them to complete and probably even succeed.


Actual Results:
The tests deadlock as soon as they hit Stream (which is almost immediately).


Regression:
This doesn’t happen if Cons’ first parameter is Box<T> instead of T.


Notes:
Pausing in the debugger shows that it’s locked somewhere inside swift_getGenericMetadata:

(lldb) bt
* thread #1: tid = 0x720d5, 0x00007fff90598132 libsystem_kernel.dylib`__psynch_cvwait + 10, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
  * frame #0: 0x00007fff90598132 libsystem_kernel.dylib`__psynch_cvwait + 10
    frame #1: 0x00007fff90efaea0 libsystem_pthread.dylib`_pthread_cond_wait + 693
    frame #2: 0x00007fff879e9c95 libc++.1.dylib`std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 47
    frame #3: 0x0000000105809ac2 libswiftCore.dylib`swift_getGenericMetadata + 562
    frame #4: 0x0000000105809c65 libswiftCore.dylib`swift_getGenericMetadata1 + 21
    frame #5: 0x00000001007276fb Traversal`create_generic_metadata + 75 at Stream.swift:0
    frame #6: 0x0000000105811299 libswiftCore.dylib`std::__1::__function::__func<swift::swift_getGenericMetadata::$_0, std::__1::allocator<swift::swift_getGenericMetadata::$_0>, (anonymous namespace)::GenericCacheEntry* ()>::operator()() + 25
    frame #7: 0x0000000105809b27 libswiftCore.dylib`swift_getGenericMetadata + 663
    frame #8: 0x0000000105809c65 libswiftCore.dylib`swift_getGenericMetadata1 + 21
    frame #9: 0x00000001007114a6 Traversal`Traversal.ReducibleOf.init <A>(reducible=(null) at 0x00007fff5fbfec08)<B : Traversal.ReducibleType>(B) -> Traversal.ReducibleOf<A> + 54 at ReducibleOf.swift:22
    frame #10: 0x000000010071e3c5 Traversal`Traversal.++ infix <A : Traversal.ReducibleType, B : Traversal.ReducibleType>(lhs=0x00007fff5fbfefe0 -> 0x0000000100711400 Traversal`partial apply forwarder for Traversal.ReducibleOf.(init <A>(Traversal.ReducibleOf<A>.Type) -> <B : Swift.GeneratorType>(() -> B) -> Traversal.ReducibleOf<A>).(closure #1) at ReducibleOf.swift, rhs=0x00007fff5fbfefc8 -> 0x0000000100711400 Traversal`partial apply forwarder for Traversal.ReducibleOf.(init <A>(Traversal.ReducibleOf<A>.Type) -> <B : Swift.GeneratorType>(() -> B) -> Traversal.ReducibleOf<A>).(closure #1) at ReducibleOf.swift) -> Traversal.ReducibleOf<A.Element> + 261 at Concat.swift:18
    frame #11: 0x000000010068bace TraversalTests`TraversalTests.ConcatTests.testInfixConcatenation (self=0x000000010030fb80)() -> () + 878 at ConcatTests.swift:15
…