Question

I think I found a bug in the libc++ list implementation. The following code will produce a compiler error (Field has incomlete type ‘foo’) when using certain build settings in xcode:

#include <list>

using namespace std;

class foo {
    public:
        list<foo> bar;
};

The settings are the following:

XCode Version: 4.4.1
C++ Language Dialect: C++11 or GNU++11
C++ Standard library: LLVM C++ standard library with C++ extensions (libc++11)

Using GCCs libstdc++ will resolve the error.
Not using the C++11 dialect will resolve the error.
Using vector instead of list will resolve the error.

I think it is a bug in the list implementation, but I am not sure.
Forgive my ignorance, but I don’t know what I should do to resolve this issue.
Switching to vector is not an option in my project and I definitely need C++11 features. That also includes shared_ptr, but the headers are missing when using GCC. Beside that, apple does not seem to provide new versions of GCC anymore.

I would very appreciate it if somebody could recreate this issue, maybe with newer header from libc++.

Also, if updating LLVM/libc++ would resolve this issue, do you recommend it?

Answer 1

C++ Standard 17.6.4.8:

In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

In particular, the effects are undefined in the following cases:

• …
• if an incomplete type (3.9) is used as a template argument when instantiating a template component,
  unless specifically allowed for that component.

None of the standard library’s container class templates, including list, mention any such allowance for an incomplete type. So your program is an invalid one that might happen to work with some compilers. That can’t be considered a bug in the standard library implementation.