a.f90

program exceptions
implicit none
integer :: s, stat

call mysum(5, s, stat)
if (stat /= 0) error stop
print *, s

call mysum(5, s)
print *, s

contains

    subroutine mysum(n, s, stat)
    integer, intent(in) :: n
    integer, intent(out), optional :: stat
    integer, intent(out) :: s
    integer, allocatable :: A(:)
    integer :: i
    call myallocate(n, A, stat=stat)
    ! We must check 'stat' by hand here and exit immediately:
    if (present(stat)) then
        if (stat /= 0) return
    end if
    do i = 1, n
        A(i) = i
    end do
    s = sum(A)
    end subroutine

    subroutine myallocate(n, A, stat)
    integer, intent(in) :: n
    integer, intent(out), optional :: stat
    integer, allocatable, intent(out) :: A(:)
    integer :: mystat
    if (present(stat)) then
        allocate(A(n), stat=stat)
    else
        allocate(A(n))
    end if
    end subroutine

end