From cppreference.com
enumclassforward_progress_guarantee{concurrent,parallel,weakly_parallel};(since C++26)forward_progress_guarantee specifies the forward progress guarantee provided by execution agents created by the execution resource associated with a scheduler.
A scheduler can report this value through std::execution::get_forward_progress_guarantee.
Member constants
Name Meaning concurrentExecution agents provide the
concurrent forward progress guarantee
: each agent eventually makes progress in a finite amount of time, independently of whether other agents make progress. parallelExecution agents provide at least the
parallel forward progress guarantee
: an agent is not required to make progress before it has started executing, but after it has executed a step, it provides the concurrent forward progress guarantee. weakly_parallelExecution agents provide only the
weakly parallel forward progress guarantee
: an agent is not guaranteed to eventually make progress, although its guarantee can be strengthened by forward progress delegation. See also