Gfortran 10 added the default behavior to make type mismatches an error instead of a warning. Legacy Fortran programs too often did not explicitly specify the procedure interfaces to allow implicit polymorphism. The Fortran 2008 standard cleaned up this situation in part with type(*).

Workaround: Gfortran flag -fallow-argument-mismatch can be used to degrade the errors to warnings. It is however strongly recommended to fix the problem in the legacy code, if it’s part of your code ownership.

Fix

For external libraries like MPI-2, the interfaces are intended to be polymorphic but use Fortran 90-style interfaces. This can be corrected from the user code by declaring an explicit interface. This specific example can be extended to other MPI-2 or other external library functions that need explicit interfaces for polymorphism.

use mpi, only : MPI_STATUS_SIZE

implicit none

interface
!! This avoids GCC >= 10 type mismatch warnings for MPI-2
subroutine mpi_send(BUF, COUNT, DATATYPE, DEST, TAG, COMM, IERROR)
type(*), dimension(..), intent(in) :: BUF
integer, intent(in) ::  COUNT, DATATYPE, DEST, TAG, COMM
integer, intent(out) :: IERROR
end subroutine

subroutine mpi_recv(BUF, COUNT, DATATYPE, SOURCE, TAG, COMM, STATUS, IERROR)
import MPI_STATUS_SIZE
type(*), dimension(..), intent(in) :: BUF
integer, intent(in) ::  COUNT, DATATYPE, SOURCE, TAG, COMM
integer, intent(out) :: STATUS(MPI_STATUS_SIZE), IERROR
end subroutine

end interface