Why std::atomic is not trivial type in only Visual C++?

Question

The Folly library requires that std::atomic<hazptr_obj*> should be a trivial type. This works with gcc and clang, but failing for Visual C++ even for std::atomic<int>. Why does std::is_trivial return false?

#include <type_traits>
#include <atomic>

static_assert(
    std::is_trivial<std::atomic<int>>::value,
    "std::atomic<int> not trivial");

Answer 1

std::atomic used to be trivial (which requires Trivially Copyable), but isn't anymore. See this answer for a great and detailed explanation for how and why that changed.

This makes VC compliant and gcc and clang non-compliant at least in C++17. As this was considered a defect by the committee, VC shows the desired behavior for C++11 and C++14, too.

For future reference, the relevant defect is DR #1734, you can see the implementation status for clang here. I'm not aware of an equivalent status page for gcc.