It looks like Past Jeff was wrong: the removal of the choice buffers from the non-ignore-TKR version of the Fortran mpi module (which is only used for gfortran <=v4.8) *does* cause ABI breakage between 1.7.3 and 1.7.4.
Per discussion on the telconf today, we decided that we're going to do this anyway, because it's an MPI-3.0 conformance issue.
I've added the following to the README in the ABI section:
* A second bug was discovered after v1.7.3 was released that causes
ABI to be broken for gfortran users who are using the "mpi" Fortran
module. In short, for versions of gfortran that do not support
"ignore TKR" functionality (i.e., gfortran <=v4.8), Open MPI was
providing interfaces for MPI subroutines with choice buffers (e.g.,
MPI_Send) in the Fortran mpi module. The MPI-3.0 specification
expressly states not to do this. To be consistent with MPI-3, Open
MPI v1.7.4 therefore removed all MPI interfaces with choice buffers
from the no-ignore-TKR version of the Fortran mpi module, even
though this breaks ABI between v1.7.3 and v1.7.4. Affected users
should be able to recompile their MPI applications with v1.7.4 with
Other Fortran compilers that provide "ignore TKR" functionality are
not affected by this change.
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