Libc++ incorrectly rejects the following code.

Reproducer from stack overflow:

// clang++ -std=c++14 test.cpp
#include <memory>

int main() {
    auto up = std::unique_ptr<int[]>(new int[5]{});
    auto sp = std::shared_ptr<int>(std::move(up));
}

I'm not yet 100% sure this is the behavior required be the C++17 standard, but It needs to be investigated and libc++ is most likely wrong.