История изменений

If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with std::call_once). Note: usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.

Первый поток, который пройдёт через объявление инициализирует. И это:

If the completion of the constructor or dynamic initialization for thread-local or static object A was sequenced-before thread-local or static object B, the completion of the destruction of B is sequenced-before the start of the destruction of A

If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with std::call_once). Note: usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.

Первый поток, который пройдёт через объявление инициализирует.