[Bug fortran/53940] New: warn about duplicate USE

[Bug fortran/53940] New: warn about duplicate USE

[Bil.Kleb at NASA dot gov]

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

             Bug #: 53940
           Summary: warn about duplicate USE
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Bil.Kleb@NASA.gov


Hi,

I'd like to see the GFortran compiler at least give a warning if a USE is
duplicated, viz,

$ cat > duplicate_use.f90 << EOF
module mod
  integer :: var = 0
end module

program duplicate_use
  use mod, only: var
  use mod, only: var ! should produce error
  print*, var
end program
EOF

$ gfortran --version | head -1
GNU Fortran (GCC) 4.6.2 20111019 (prerelease)

$ gfortran -W -Wall -Wextra -pedantic-errors duplicate_use.f90
[no complaints]

The old DEC Alpha compiler does:

alpha% fort -what
    Compaq Fortran V1.2.0-1882
    Compaq Fortran Compiler V1.2.0-1882-48BBF

alpha% fort -c duplicate_use.f90
f90: Warning: duplicate_use.f90, line 6: Conflicting attributes or multiple
declaration of name.   [VAR]
  use mod, only: var
-----------------^
f90: Error: duplicate_use.f90, line 8: The same named entity from different
modules and/or program units cannot be referenced.   [VAR]
  print*, var
----------^

Thanks for consideration,
--
Bil

[Bug fortran/53940] warn about duplicate USE

[burnus at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |burnus at gcc dot gnu.org

--- Comment #1 from Tobias Burnus <burnus at gcc dot gnu.org> 2012-07-12 13:08:51 UTC ---
(In reply to comment #0)
> The old DEC Alpha compiler does:
[...]
> f90: Error: duplicate_use.f90, line 8: The same named entity from different
> modules and/or program units cannot be referenced.   [VAR]
>   print*, var
> ----------^

That that one is completely bogus. The standard allows multiple inclusion of
the same variable; and your variable is - contrary to the error message - from
the same module. For instance, the following is valid:

  use mod
  use mod, var2 => var

That way one has use-associated the variable "var" under the name "var" and
under the name "var2".


The Fortran standard (here: Fortran 2008, "11.2.2 The USE statement and use
association") has:

"More than one USE statement for a given module may appear in a specification
part. If one of the USE statements is without an ONLY option, all public
entities in the module are accessible. If all the USE statements have ONLY
options, only those entities in one or more of the only-lists are accessible.

"An accessible entity in the referenced module has one or more local
identifiers. [...]" (Cf. http://gcc.gnu.org/wiki/GFortranStandards)


Hence, I am inclined to reject the warning proposal. (The request to print an 
error is simply invalid.)

However, if you can give a good argument why you want to have a special warning
and when it exactly should be triggered, one could consider it. However, I
currently fail to see a real use case for that.

Note: module entities which have been explicitly use associated but aren't used
are warned for. Thus, one could consider to warn if the same entity under the
same name is multiple times use associated. However, that's not only valid but
seems to be also harmless and a rather special case.

[Bug fortran/53940] warn about duplicate USE

[Bil.Kleb at NASA dot gov]

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

--- Comment #2 from Bil Kleb <Bil.Kleb at NASA dot gov> 2012-07-12 13:25:29 UTC ---
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 ! should produce at least a warning?
  var = 1
  print*, var
end program
EOF

% gfortran duplicate_declaration.f90 
duplicate_declaration.f90:3.16:

  integer :: var ! should produce at least a warning?
                1
Error: Symbol 'var' at (1) already has basic type of INTEGER

While the error is mostly harmless, we find it useful to know about to keep our
code clean.

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

[burnus at gcc dot gnu.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.

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

[Joost.VandeVondele at mat dot ethz.ch]

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

Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Joost.VandeVondele at mat dot ethz
                   |                            |.ch

--- Comment #4 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
still useful.

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

[dominiq at lps dot ens.fr]

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

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-10-13
     Ever confirmed|0                           |1

--- Comment #5 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
No warning at r228753.

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

[dominiq at lps dot ens.fr]

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

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #7 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
Closing.