Comment (by Roland Haas):

The workaround

sudo port -N uninstall gcc9
sudo port -sN install gcc9

is no longer needed for MacPorts 2.6.2 (2019-11-11) on macOS Catalina, but still required on Mojava.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2290/failure-to-compile-et_mayer-using-gcc-mp-9