[Bug fortran/91556] Problems with better interface checking

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91556

--- Comment #19 from anlauf at gcc dot gnu.org ---
(In reply to Thomas Koenig from comment #18)
> (In reply to anlauf from comment #14)
> > The current solution is a bit annoying for implicitly-derived interfaces.
> > 
> > Consider a code like:
> > 
> > module foo
> >   implicit none
> >   type t1
> >      integer :: i = 1
> >   end type t1
> >   type t2
> >      integer :: j = 2
> >   end type t2
> > contains
> >   subroutine s1 (x)
> >     type(t1) :: x
> >     call my_mpi_bcast_wrapper (x, storage_size (x)/8)
> >   end subroutine s1
> >   subroutine s2 (y)
> >     type(t2) :: y
> >     call my_mpi_bcast_wrapper (y, storage_size (y)/8)
> >   end subroutine s2
> > end module foo
> > 
> > That's perfectly legal,
> 
> This is illegal, as far as I know. The type names are different,
> which makes them different types.

Of course the types are different - that's the point!

The above is an attempt to extract a self-contained example demonstrating
what does happen in real-world codes using MPI.  You can convert it to the
real thing yourself (see e.g. man mpi_bcast).

What I wanted to say: in many codes using MPI, the 'buffer' argument of MPI_*
can be of any type or kind, provided the other arguments are appropriately
set.  There is no explicit interface - the library routines may not even be
written in Fortran - and there cannot be any reasonable inference from an
implicit interface.  Using this to generate an error *by default* is not a
good idea. 

The same problem shows up for me in parts of the code using direct calls to
BLAS (again, no explicit interfaces), or to NetCDF.  I guess this is similar
to the issues people reported for the SPEC codes.

The recommendation to use -fallow-argument-mismatch to downgrade this to a
warning (not silence it!) does not really solve the issue: the signal-to-noise
ratio of gfortran warnings has become close to useless.  (Just ask colleagues
whether they pay attention to warnings.)

And do you really want users to add -w to the list of options?
If you think about it, you might conclude that this will do gfortran a
poor service.

I'll stop ranting now.