[Bug c/96571] New: Bad "set but not used" warning with _Generic

[Bug c/96571] New: Bad "set but not used" warning with _Generic

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

            Bug ID: 96571
           Summary: Bad "set but not used" warning with _Generic
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

Compile the following with -Wall:

enum E { V };

int
f (void)
{
  enum E v;
  return _Generic (v, enum E : 0);
}

This produces a warning:

t.c: In function 'f':
t.c:6:10: warning: variable 'v' set but not used [-Wunused-but-set-variable]
    6 |   enum E v;
      |          ^

It's arguable whether the variable is "used" when it's only referenced in the
controlling expression of _Generic (so never actually evaluated), but it's
certainly not set.

[Bug c/96571] Bad "set but not used" warning with _Generic

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
You should then mark_exp_read the _Generic controlling expression.

[Bug c/96571] Bad "set but not used" warning with _Generic

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-08-13
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 49057
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49057&action=edit
gcc11-pr96571.patch

Untested fix.

[Bug c/96571] Bad "set but not used" warning with _Generic

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:6d42cbe5ad7a7b46437f2576c9920e44dc14b386

commit r11-2732-g6d42cbe5ad7a7b46437f2576c9920e44dc14b386
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Aug 18 07:51:58 2020 +0200

    c: Fix -Wunused-but-set-* warning with _Generic [PR96571]

    The following testcase shows various problems with -Wunused-but-set*
    warnings and _Generic construct.  I think it is best to treat the selector
    and the ignored expressions as (potentially) read, because when they are
    parsed, the vars in there are already marked as TREE_USED.

    2020-08-18  Jakub Jelinek  <jakub@redhat.com>

            PR c/96571
            * c-parser.c (c_parser_generic_selection): Change match_found from
bool
            to int, holding index of the match.  Call mark_exp_read on the
selector
            expression and on expressions other than the selected one.

            * gcc.dg/Wunused-var-4.c: New test.

[Bug c/96571] Bad "set but not used" warning with _Generic

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:5c16de18f26df961e00a049f198aeb9dce8de225

commit r10-8668-g5c16de18f26df961e00a049f198aeb9dce8de225
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Aug 18 07:51:58 2020 +0200

    c: Fix -Wunused-but-set-* warning with _Generic [PR96571]

    The following testcase shows various problems with -Wunused-but-set*
    warnings and _Generic construct.  I think it is best to treat the selector
    and the ignored expressions as (potentially) read, because when they are
    parsed, the vars in there are already marked as TREE_USED.

    2020-08-18  Jakub Jelinek  <jakub@redhat.com>

            PR c/96571
            * c-parser.c (c_parser_generic_selection): Change match_found from
bool
            to int, holding index of the match.  Call mark_exp_read on the
selector
            expression and on expressions other than the selected one.

            * gcc.dg/Wunused-var-4.c: New test.

    (cherry picked from commit 6d42cbe5ad7a7b46437f2576c9920e44dc14b386)

[Bug c/96571] Bad "set but not used" warning with _Generic

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:d2472e513c6490b6e2832b07d08731df26e6dc06

commit r9-8909-gd2472e513c6490b6e2832b07d08731df26e6dc06
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Aug 18 07:51:58 2020 +0200

    c: Fix -Wunused-but-set-* warning with _Generic [PR96571]

    The following testcase shows various problems with -Wunused-but-set*
    warnings and _Generic construct.  I think it is best to treat the selector
    and the ignored expressions as (potentially) read, because when they are
    parsed, the vars in there are already marked as TREE_USED.

    2020-08-18  Jakub Jelinek  <jakub@redhat.com>

            PR c/96571
            * c-parser.c (c_parser_generic_selection): Change match_found from
bool
            to int, holding index of the match.  Call mark_exp_read on the
selector
            expression and on expressions other than the selected one.

            * gcc.dg/Wunused-var-4.c: New test.

    (cherry picked from commit 6d42cbe5ad7a7b46437f2576c9920e44dc14b386)

[Bug c/96571] Bad "set but not used" warning with _Generic

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96571

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
For 8.5 fixed in r8-10512-g2346f2b89780790210816eeea12b817e61652b26 , for 9.4+
and 10.3+ and 11.1+ by the above commits.