[Patch, Fortran] PR48858/55465 - permit multiple bind(C) declarations (but not definitions) for the same proc

[Tobias Burnus]

Followup (and depending on) to the C binding patches for
* COMMON: http://gcc.gnu.org/ml/fortran/2013-05/msg00048.html
* Procedures: http://gcc.gnu.org/ml/fortran/2013-05/msg00051.html
which honour Fortran 2008, where the Fortran name is no longer a global 
identifier if a binding name has been specified.

The main reason for this patch is a build failure of Open MPI (requires 
!gcc$ attributes no_arg_check, i.e. it only affects GCC 4.9). Open MPI 
uses somethine like:
   interface
     subroutine pmpi_something() bind(C,name="MPI_something")
   ...
and in a different module:
   interface
     subroutine mpi_something() bind(C,name="MPI_something")
   ...

Currently, gfortran rejects it because it only permits one 
definition/declaration per translation unit. However, there is no reason 
why multiple INTERFACE blocks shouldn't be permitted.


Remarks:

a) Better argument checks if definition and declaration are in the same 
file. (see INTENT patch in a test case)

b) Currently, no check is done regarding the characteristic of procedure 
declarations. Of course, the declaration has to be compatible with the C 
procedure. However, there seems to be the wish* to permit compatible 
input - even if the Fortran characteristic is different. For instance 
"int *" takes both a scalar integer ("int i; f(&i)") and arrays ("int 
i[5]; f(i)"). Or also popular according to the PRs: Taking a C_LOC or an 
integer(c_intptr_t).

(* Seemingly, also J3 and/or WG5 discussed this (plenum? subgroups?) and 
they had the permit it. However, finding some official document is 
difficult.)

I was wondering for a while what should be permitted and what shouldn't, 
but I have now decided to put that completely into the hands of the user.


Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias
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