Comment (by Roland Haas):

The macports ticket suggests adding +ld64_xcode to the options for the ld64 package (however it did not seem to have helped there, installed gcc9 from source did). We used to have ld64_xcode as well (b/c one of Helvi’s students found it helped them) but removed it again in git hash 69ff3a5 "CactusTutorial: update installation instructions for packages" of jupyter-et where it is claimed that it was not required after all given a fully installed Xcode package.

The system where the error listed above (I am the reporter) happens may have a messy XCode installation (though XCode is present and the command line tools are present as well).

If +ld64_xcode helps I will add it back to the tutorial instructions since it would seem harmless if present.

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