alt Hacker News`std::accumulate` is defined to have sequential semantics, so the analysis required to make it parallel is probably not that different than starting from the loop version. I guess you could have an alternate `accumulate_associative` that uses the same interface but assumes the reduction is associative and has unspecified evaluation order?
C++ has std::reduce for that, which is std::accumulate except it's defined to operate without any specific ordering.