https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91731
--- Comment #5 from Damian Rouson <damian at sourceryinstitute dot org> --- MPICH must find ISO_Fortran_binding.h in order to build the modern Fortran bindings that the MPI standard provides through the "mpi_f08" Fortran module. Gfortran only started providing ISO_Fortran_binding.h in release 9.1. In fact, ISO_Fortran_binding.h was primarily motivated by the need for modern MPI bindings. Hopefully someone on the MPICH project will implement more modern bindings now. I'll ask when I report this.
