Comment 2 for bug 2030797

See https://github.com/llvm/llvm-project/issues/63447

The problem is the following:

std::vector<int> x(n), y(n);
auto ids = std::views::iota((int)0, (int)x.size());
std::for_each(std::execution::par_unseq, ids.begin(), ids.end(), [x = x.data(), y = y.data()](int i) {
    x[i] = y[i];
});

Iterators from C++20 ranges model the C++20 random_access_iterator concept, but do not necessarily have a random access iterator tag. They are not recognized by the PSTL as random access iterators (but forward iterators), causing the parallel algorithms to fall back to sequential execution.

This is significantly impacting the performance a couple of large HPC applications.

A quick and dirty workaround is to modify pstl/executors_impls.hpp by changing the random_access_iterator<IteratorType> to:

template <typename _IteratorType>
struct __is_random_access_iterator<_IteratorType> {
    static constexpr bool value = (
        (bool)std::is_same_v<typename std::iterator_traits<_IteratorType>::iterator_category, std::random_access_iterator_tag>
        || (bool)::std::random_access_iterator<_IteratorType>
    );
    typedef std::integral_constant<bool, value> type;
};

Since llvm-project/pstl has been forked by libc++, it does no longer make sense to try to patch pstl upstream to fix this issue.