@eddelbuettel Could we fix this? The issue arises only with OneTBB backend and Clang with libc++ on < 10.13 (that is why we did not face it earlier: legacy TBB was used until recently).

We could fix it for dependents of RcppParallel like this:
macports/macports-ports@627f0dd

But considering a number of affected ports, it is somewhat suboptimal solution.

(To be explicit, this should not be used with gcc and libstdc++ on any macOS versions, including < 10.13. Everything works fine there, and also the flag is Clang-specific.)