[Note 2:

Implementations must ensure that internal synchronization inside standard library functions does not prevent forward progress when those functions are executed by threads of execution with weakly parallel forward progress guarantees.

— end note]

[Example 2: int x = 0; std::mutex m; void f() { int a[] = {1,2}; std::for_each(std::execution::par_unseq, std::begin(a), std::end(a), [&](int) { std::lock_guard<mutex> guard(m); ++x; }); }

The above program may result in two consecutive calls to m.lock() on the same thread of execution (which may deadlock), because the applications of the function object are not guaranteed to run on different threads of execution.

— end example]