std::atomic_flag::operator=

From cppreference.com
< cpp‎ | atomic‎ | atomic flag
Defined in header <atomic>
atomic_flag& operator=( const atomic_flag& ) = delete;
(1) (since C++11)
atomic_flag& operator=( const atomic_flag& ) volatile = delete;
(2) (since C++11)

std::atomic_flag is not assignable, its assignment operators are deleted.