How is std::mutex's constexpr constructor implemented?

While looking at the C++ Reference for std::mutex, I noticed that the constructor for std::mutex is marked constexpr.

This is surprising at first, since we usually have to make a system call (either pthread_mutex_init() (POSIX) or CreateMutex() (Windows) to initialize a mutex. However, on closer inspection, for POSIX, one can use the constant PTHREAD_MUTEX_INITIALIZER to statically initialize the mutex (perhaps as a global variable), though I can find no equivalent for Windows.

However, even if this static initialization for POSIX was the reason behind the constexpr constructor, there're still various issues with an implementation:

1. On Windows (and perhaps other non-POSIX systems), there may not be a way to statically initialize mutexes.
2. It was impossible to have different code paths depending on whether the constructor is being called at compilation time until C++20's std::is_constant_evaluated() was added, so we don't have a way to determine if PTHREAD_MUTEX_INITIALIZER or pthread_mutex_init() should be used.

So, how does one implement the constexpr constructor for std::mutex?