std::atomic<T>::is_always_lock_free (3) - Linux Man Pages
static constexpr bool is_always_lock_free = /*implementation-defined*/; (since C++17)
Equals true if this atomic type is always lock-free and false if it is never or sometimes lock-free.
The value of this constant is consistent with both the macro ATOMIC_xxx_LOCK_FREE, where defined, with the member function is_lock_free and non-member function std::atomic_is_lock_free.
There is no non-member function equivalent of this static member constant because non-member functions take pointers to atomic types, and therefore aren't as useful in constant_expressions.
is_lock_free (public member function)
atomic_is_lock_free checks if the atomic type's operations are lock-free