g95 vs gfortran : Dummy procedure 'f' of PURE procedure at (1) must also be PURE

Question

  ....
  abstract interface
     pure function fi(t,u) result (fu)
       use NumberKinds
       real(kp), dimension(:), intent(in) :: u
       real(kp), intent(in) :: t
       real(kp), dimension(size(u)) ::  fu
     end function fi
  end interface


contains

  pure function rk4_step(u,f,dt) result(un)
    use NumberKinds
    real(kp), intent(in) :: dt
    real(kp), intent(in),  dimension(:) :: u
    real(kp), dimension(size(u))  :: k1,k2,k3,k4,un
    procedure(fi) :: f
    integer :: N
    ...

  end function rk4_step
  ...

I get this error message with g95: G95 (GCC 4.0.3 (g95 0.94!) Jan 17 2013)

In file src/integrators.f95:34

  pure function rk4_step(u,f,dt) result(un)
                           1
Error: Dummy procedure 'f' of PURE procedure at (1) must also be PURE
Makefile:28: recipe for target 'test_rk4' failed

I don't get it with gfortran: GNU Fortran (GCC) 4.8.2 20140206 (prerelease) and the program compiles without further problems

Answer 1

Just a compiler bug. g95 is very old, when Fortran 2003 just started to be implemented in compilers. It has decent C interop, stream access and so on, but many other features are not implemented at all.

Don't use a compiler, that has not been updated for a long time (with a small exception recently) if you need modern features support.

You can try an interface block inside rk4_step instead of the abstract interface, that is Fortran 90 and works for me in g95.

  pure function rk4_step(u,f,dt) result(un)
    use NumberKinds
    real(kp), intent(in) :: dt
    real(kp), intent(in),  dimension(:) :: u
    real(kp), dimension(size(u))  :: k1,k2,k3,k4,un
    integer :: N

    interface
       pure function f(t,u) result (fu)
         use NumberKinds
         real(kp), dimension(:), intent(in) :: u
         real(kp), intent(in) :: t
         real(kp), dimension(size(u)) ::  fu
       end function f
    end interface
  end function rk4_step