[Bug fortran/53940] Optionally warn about multiple explicit USE-association of the same symbol

["burnus at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53940

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
            Summary|warn about duplicate USE    |Optionally warn about
                   |                            |multiple explicit
                   |                            |USE-association of the same
                   |                            |symbol

--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> 2012-07-12 13:44:55 UTC ---
(In reply to comment #2)
> I guess I see the USE ONLY as similar to a declaration, and to have two of the
> same declarations in a program is an error, e.g.,
> $ cat duplicate_declaration.f90 << EOF
> program duplicate_declaration
>   integer :: var
>   integer :: var

Well, here you declare two different variables with the same name. In the
original example, you use associate the same variable under the same name.
Thus, technically, this example is invalid while the first one is valid.
(At least that's how I understand the Fortran standard.)


However, I concur that one could warn if the same variable appears multiple
times in a "USE ..., ONLY:" either as you had:

  use mod, only: var
  use mod, only: var
or as in
  use mod, only: var, var

Both are perfectly valid, but I concur that (in well written code) it should
not occur. I don't think we want to warn for:
  use mod
  use mod, only: var
which also (kind of) use-associates "var" multiple times.


The warning implementation should be doable somewhere in the depths of module.c
as we only set "sym->attr.use_only" for explicitly use-associated symbols.