Unity build is broken

[tamiko]

For the upcoming 9.4 release we should fix our unity build again :-)

https://cdash.dealii.43-1.org/viewBuildError.php?buildid=2195

[kronbichler]

I cannot reproduce on my two configurations, one with gcc-11, MPI-3.1, no Trilinos, no PETSc, the other with clang-14, MPI-3.1, Trilinos, PETSc but none of the more special libraries. It must be a combination of some particular library/feature or compiler.

[drwells]

I can reproduce this - I am working on a fix.

[drwells]

https://github.com/petsc/petsc/blob/27c9ef7be839ca1f58a90cb820253679fac370b3/include/petsclog.h#L460-L462

defines

#define MPI_Send_c(buf,count,datatype,dest,tag,comm) \
  ((petsc_send_ct++,0) || PetscMPITypeSize((count),(datatype),(&petsc_send_len)) || MPI_Send_c((buf),(count),(datatype),(dest),(tag),(comm)))

so its not possible to define a function with that name after including a PETSc header - we will always get errors resulting from macros. More generally, the big mpi library is not compatible with PETSc for this reason so this problem extends past the issue of unity builds.

This snippet always fails to compile:

#include <petsclog.h>

#include <deal.II/base/mpi_large_count.h>

int main() {}

[bangerth]

Ugh, what a moronic system for them to use...