Different interfaces cause problem in 4.5, but not 4.4

[Tobias Burnus]

  Dear Paul,

Paul van Delst wrote:
> Please consider the following code:
>
>    subroutine esmf_gridcompsetservices (subroutinename)
>      implicit none
>      interface
>        subroutine subroutinename (rc)
>          integer :: rc
> [...]
>    subroutine nems_register(rc)
>      integer,intent(out) :: rc
> [...]
>    call esmf_gridcompsetservices(nems_register)

> When compiled with gfortran 4.5 we get the following:
>
>   call esmf_gridcompsetservices(nems_register)
>                                 1
> Error: Interface mismatch in dummy procedure 'subroutinename' at (1): INTENT mismatch in argument 'rc'
>
> My initial reaction when I was shown this code, and told gfortran 4.5 wouldn't compile it, was that the interfaces were different so it shouldn't compile.
>
> But then when I tried gfortran 4.4 it worked. And the test code compiles and runs as expected with g95, pgf95, ifort (on linux) and xlf95 on AIX.

If you look for another compiler which does not like the code, try NAG:

Error: line 26: Dummy proc SUBROUTINENAME arg 1 has different INTENT 
from actual proc NEMS_REGISTER arg
Error: line 26: Incompatible procedure argument for SUBROUTINENAME (no. 
1) of ESMF_GRIDCOMPSETSERVICES

As both NAG and gfortran 4.5/4.6 try to tell you: Passing a procedure 
with having an INTENT specifies as actual argument to a dummy procedure 
with arguments which do not have an INTENT is invalid. Or to quote the 
standard (from Fortran 2008, which meanwhile has been published by ISO):

"12.5.2.9 Actual arguments associated with dummy procedure entities"
[...]
"If the interface of a dummy procedure is explicit, its characteristics 
as a procedure (12.3.1) shall be the same as those of its effective 
argument, except that a pure effective argument may be associated with a 
dummy argument that is not pure and an elemental intrinsic actual 
procedure may be associated with a dummy procedure (which cannot be 
elemental)."

And jumping to 12.3.1 one finds:

"The characteristics of a procedure are [...] the characteristics of its 
dummy arguments,"
[...]
"The characteristics of a dummy data object are [...] its intent 
(5.3.10, 5.4.9),"

As those are no constraints (C<number>), the compiler does not have to 
diagnose it, but - as Richard Maine usually puts it - it is a matter of 
quality of implementation.

Tobias,
who has already found a bug in a program due to this checking where a 
procedure with an INTENT(OUT) dummy was passed to a dummy procedure with 
an INTENT(IN) dummy argument.

PS: Do not underestimate the importance of INTENT for the program 
correctness. Especially if one uses INTENT(IN), gfortran does all kind 
of optimizations which leads to wrong code if the argument is modified. 
I think there are not yet any optimizations done in the caller for 
INTENT(OUT), but that might come.