Cannot complete container and smart pointer with '-std=c++11' flag

[zarvd]

I add C++11(or C++1y) flag

(setq ac-clang-cflags (list "-std=c++11"))

and then

std::  // it works! It shows candidates' menu
std::vector<int> c;
c.   // But this is not working

class A {
public:
    int value;
};

std::unique_ptr<A> a(new A);
a->  // It is not working either

And what if I remove the C++11(or C++1y) flag, everything works well. I have tried to track it out, and found clang_codeCompleteAt()return null result while completing container or smart pointer. My clang version is 3.7.0 (Is that a bug in clang?)