Hi,

I hope that these citations are correct in relation to the official Fortran 2003 standard. Sure, it all is more or less right.

Based on this, for use mpi and use mpi3, only one identical backend (written in C) should be necessary. OK, fine, but that is not implied by the words you quoted. For example, it is perfectly possible (plausible) for an integer and a struct to be passed differently (e.g., integer goes in register, the struct on the stack), and adding SEQUENCE does nothing to change that. If you are relying on some "backend" (meaning, really, ABI) property, say so, do not pretend that the Fortran standard guarantees something it does not.

This is not to say that it will not be possible to still use one backend even if the interface says TYPE(MPI_Comm) instead of INTEGER---but that is outside the scope of the Fortran standard. One should instead look at ABIs and conventions compilers use to pass such arguments around. Best, Aleks