<atomic>: In pre-C++20 mode, the constructor should be trivial

Starting from VS 2019 16.6 (#336), std::atomic constructor is unconditionally initializing the value:

    constexpr atomic() noexcept(is_nothrow_default_constructible_v<_Ty>) : _Base() {}

Before this version. the constructor was trivial instead:

#ifdef __clang__ // TRANSITION, VSO-406237
    constexpr atomic() noexcept(is_nothrow_default_constructible_v<_Ty>) : _Base() {}
#else // ^^^ no workaround / workaround vvv
    atomic()                                  = default;
#endif // TRANSITION, VSO-406237

As far as I understand, this is a behavior change in C++20 standard. However, I would not expect to see this behavior without specifying /std:c++latest.

This is significant because it results in some cases where std::atomic in global scope now generates dynamic initializers. Notably, in this case:

struct Foo
{
    std::atomic<int> data[2];
};

Foo foo[2];

Before this change, the foo variable was placed into BSS with no dynamic initializer. After this change, the dynamic initializer is emitted. This is problematic because if any code in static constructors accesses the atomic, it may work on the atomic before it's initialized which means that the initialization will overwrite the value of the atomic to 0.

Thus this is a breaking change - which would be fine except that it's a silent breaking change, and I'd expect to have to opt into this change by using the latest standard version instead.

Review the constructor change shown in the issue alongside issue #336 and the closed pull request #694. Reproduce the global Foo array example in pre-C++20 mode and compare whether it receives a dynamic initializer; done means the constructor preserves the expected trivial behavior without requiring the latest language mode.