API Blueprint with empty One Of entry causes segmentation fault

[kylef]

The following API Blueprint which has an empty One Of entry causes segmentation fault:

# GET /

+ Request (application/json)
    + Attributes
        + One Of
            + 

$ drafter document.apib
terminated by signal SIGSEGV (Address boundary error)

$ drafter --version
v4.0.0-pre.5

Process 98591 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x000000010005c6e9 drafter`refract::dsd::Option::Option(refract::dsd::Option const&) [inlined] refract::dsd::Option::Option(el=0x0000000102345f00)::$_0::operator()(std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const&) const at Option.cc:52:24 [opt]
   49  	        std::back_inserter(elements_),
   50  	        [](const value_type& el) -> std::unique_ptr<IElement> {
   51  	            assert(el);
-> 52  	            return el->clone();
   53  	        });
   54  	}
   55
Target 0: (drafter) stopped.

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x000000010005c6e9 drafter`refract::dsd::Option::Option(refract::dsd::Option const&) [inlined] refract::dsd::Option::Option(el=0x0000000102345f00)::$_0::operator()(std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const&) const at Option.cc:52:24 [opt]
    frame #1: 0x000000010005c6e6 drafter`refract::dsd::Option::Option(refract::dsd::Option const&) [inlined] std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> >, std::__1::allocator<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > > > > std::__1::transform<std::__1::__wrap_iter<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const*>, std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> >, std::__1::allocator<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > > > >, refract::dsd::Option::Option(__first=<unavailable>, __last=<unavailable>, __result=<unavailable>)::$_0>(std::__1::__wrap_iter<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const*>, std::__1::__wrap_iter<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const*>, std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> >, std::__1::allocator<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > > > >, refract::dsd::Option::Option(refract::dsd::Option const&)::$_0) at algorithm:1963 [opt]
    frame #2: 0x000000010005c6da drafter`refract::dsd::Option::Option(this=0x00007ffeefbfe378, other=<unavailable>) at Option.cc:47 [opt]
    frame #3: 0x000000010005d151 drafter`refract::Element<refract::dsd::Option>::clone(this=<unavailable>, flags=15) const at Element.h:147:29 [opt]
    frame #4: 0x000000010005cafc drafter`refract::dsd::Select::Select(refract::dsd::Select const&) [inlined] std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > refract::clone<refract::Element<refract::dsd::Option>, std::__1::enable_if<true, void> >(el=<unavailable>, flags=15) at ElementIfc.h:126:68 [opt]
    frame #5: 0x000000010005caef drafter`refract::dsd::Select::Select(refract::dsd::Select const&) [inlined] refract::dsd::Select::Select(el=0x0000000102345760)::$_0::operator()(std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > const&) const at Select.cc:54 [opt]
    frame #6: 0x000000010005cae7 drafter`refract::dsd::Select::Select(refract::dsd::Select const&) [inlined] std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > >, std::__1::allocator<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > > > > std::__1::transform<std::__1::__wrap_iter<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > const*>, std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > >, std::__1::allocator<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > > > >, refract::dsd::Select::Select(__first=<unavailable>, __last=<unavailable>, __result=<unavailable>)::$_0>(std::__1::__wrap_iter<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > const*>, std::__1::__wrap_iter<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > const*>, std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > >, std::__1::allocator<std::__1::unique_ptr<refract::Element<refract::dsd::Option>, std::__1::default_delete<refract::Element<refract::dsd::Option> > > > > >, refract::dsd::Select::Select(refract::dsd::Select const&)::$_0) at algorithm:1963 [opt]
    frame #7: 0x000000010005cadb drafter`refract::dsd::Select::Select(this=0x00007ffeefbfe468, other=<unavailable>) at Select.cc:48 [opt]
    frame #8: 0x000000010005b13f drafter`refract::Element<refract::dsd::Select>::clone(this=<unavailable>, flags=15) const at Element.h:147:29 [opt]
    frame #9: 0x000000010005be85 drafter`refract::dsd::Object::Object(refract::dsd::Object const&) [inlined] refract::dsd::Object::Object(el=<unavailable>)::$_0::operator()(std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const&) const at Object.cc:47:24 [opt]
    frame #10: 0x000000010005be74 drafter`refract::dsd::Object::Object(refract::dsd::Object const&) [inlined] std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> >, std::__1::allocator<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > > > > std::__1::transform<std::__1::__wrap_iter<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const*>, std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> >, std::__1::allocator<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > > > >, refract::dsd::Object::Object(__first=<unavailable>, __last=<unavailable>, __result=<unavailable>)::$_0>(std::__1::__wrap_iter<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const*>, std::__1::__wrap_iter<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > const*>, std::__1::back_insert_iterator<std::__1::vector<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> >, std::__1::allocator<std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > > > >, refract::dsd::Object::Object(refract::dsd::Object const&)::$_0) at algorithm:1963 [opt]
    frame #11: 0x000000010005be68 drafter`refract::dsd::Object::Object(this=0x00007ffeefbfe548, other=<unavailable>) at Object.cc:42 [opt]
    frame #12: 0x00000001000448ab drafter`refract::Element<refract::dsd::Object>::clone(this=<unavailable>, flags=15) const at Element.h:147:29 [opt]
    frame #13: 0x000000010000ef74 drafter`PayloadToRefract(drafter::NodeInfo<snowcrash::Payload> const&, drafter::NodeInfo<snowcrash::Action> const&, drafter::ConversionContext&) [inlined] std::__1::unique_ptr<refract::IElement, std::__1::default_delete<refract::IElement> > refract::clone<refract::IElement, std::__1::enable_if<true, void> >(el=<unavailable>, flags=15) at ElementIfc.h:126:68 [opt]

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