[求助] error #6404: This name does not have a type, and must have an explicit type. ...

program main
    implicit none

    integer, parameter :: nvar = 2, nstep=500
    real(kind=8), dimension(nvar) :: ystart=[0.0d0,0.0d0]
    real(kind=8), dimension(nstep) :: xx
    real(kind=8), dimension(nvar,nstep) :: y, ytrue

    real(kind=8) :: x1=0.0d0, x2=10.0d0
    integer :: i

    call rkdub(Qa,xx, y, ystart, nvar, x1, x2, nstep, diff)                                               !这一行出现：error #6404: This name does not have a type, and must have an explicit type.   [QA]

    print*,y(1,i), y(2,i)
end program
    ! Define the Ordinary Differential Equations(ODEs) in this fileubroutine diff(Qa, t, y, dydx)
   implicit none

   integer, parameter :: n = 2
   real(kind=8) :: Qa, t
   real(kind=8) :: y(n), dydx(n)
   real(kind=8) :: Kr=1.4
   real(kind=8) :: Pg=5000000
   real(kind=8) :: Vg=0.04
   !real(kind=8), parameter :: Qa=0.0005
   real(kind=8) :: Dm=0.00025
   real(kind=8) :: Jm=2
   real(kind=8) :: Ro=850
   real(kind=8) :: Cd=0.61
   real(kind=8) :: Af=0.0005
   real(kind=8) :: Bg=2
   ! Define the ODE or ODEs here   
   !dx(1) = Kr*Pg*(Qa-Dm*x(2))/Vg
   !dx(2) = (x(2)*x(2)*Ro*Dm*Dm*Dm/(2*(Cd*Af)*(Cd*Af))-Bg*x(2)+x(1)*Dm)/J
   dydx(1) = Qa-Dm*y(2)
   !dx(2) = (Dm*Pg/((1-x(1)/Vg)**1.4)-x(2)*x(2)*Ro*Dm*Dm*Dm/(2*(Cd*Af)*(Cd*Af))-Bg*x(2))/Jm
   dydx(2) = (Dm*(Vg**Kr*Pg/((Vg-y(1))**1.4))-(y(2)*y(2)*Ro*Dm*Dm*Dm)/(2*(Cd*Af)*(Cd*Af))-Bg*y(2))/Jm
end subroutine diff

! RK4 numerical solution of differential equations
subroutine ODE_RK4(Qa,t, y, n, h, yout, diff)
    implicit none
    external :: diff
    integer n
    real(kind=8) :: Qa, t, h
    real(kind=8) :: y(n)
    real(kind=8) :: yout(n)
    real(kind=8) :: k1(n),k2(n),k3(n),k4(n)
    ! Calculate k1
    call diff(Qa, t, y, dydx)                                                                   ！这一行出现：error #6404: This name does not have a type, and must have an explicit type.   [QA]
    k1 = h*dydx
    call diff(Qa, t+0.5d0*h, y+0.5*k1, dydx)
    k2 = h*dydx
    call diff(Qa, t+0.5d0*h, y+0.5*k2, dydx)
    k3 = h*dydx
    call diff(Qa, t+h, y+k3, dydx)
    k4 = h*dydx

    yout = y + k1/6.0d0 + k2/3.0d0 + k3/3.0d0 + k4/6.0d0
end subroutine ODE_RK4

subroutine rkdub(Qa,xx, y, ystart, nvar, x1, x2, nstep, diff)
    implicit none
    integer, intent(in) :: nvar, nstep, Qa
    real(kind=8), intent(in) :: x1, x2
    real(kind=8), dimension(nvar), intent(in) :: ystart
    real(kind=8), dimension(nstep), intent(out) :: xx
    real(kind=8),dimension(nvar,nstep), intent(out) :: y
    external :: diff

    real(kind=8) :: h, x
    integer :: i,j

    x = x1
    h = (x2-x1) / nstep
    xx(1) = x1
    y(:,1) = ystart
    do i=2, nstep
        call ODE_RK4(Qa,x, y(:,i-1), nvar, h, y(:,i), diff)
        if (x+h == x) then
        write(*,*) 'stepsize not significant in rkdub'
    end if
    x = x + h
    xx(i) = x
    end do
end subroutine rkdub