In this CI run (linux-clang-avg):

  https://gitlab.com/petsc/petsc/-/jobs/1260342204

The compile fails building a C++ file, demangle.cxx. It fails at the first line,
including <petsc/private/petscimpl.h>, down in petscsystypes.h. It bombs
during the definition of complex because it looks like the compiler is not
defining __cplusplus, and thus takes the wrong branch. Is this what is happening?
I cannot access this machine.

  Thanks,

      Matt

--
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener