[Bug c++/66028] New: false positive, unused loop variable

[Bug c++/66028] New: false positive, unused loop variable

[ncm at cantrip dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66028

            Bug ID: 66028
           Summary: false positive, unused loop variable
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ncm at cantrip dot org
  Target Milestone: ---

struct range {
    int start; int stop;
    struct iter {
        int i;
        bool operator!=(iter other) { return other.i != i; };
        iter& operator++() { ++i; return *this; };
        int operator*() { return i; }
    };
    iter begin() { return iter{start}; }
    iter end() { return iter{stop}; }
};
int main()
{
   int power = 1;
   for (int i : range{0,10})
       power *= 10;
}
bug.cc: In function ‘int main()’:
bug.cc:15:13: warning: unused variable ‘i’ [-Wunused-variable]
    for (int i : range{0,10})

Manifestly, i is used to count loop iterations.  The warning cannot be
suppressed by any decoration of the declaration; the best we can do is

  void(i), power *= 10;

in the loop body.  The warning is useful in most cases.  The exception might be
that, here, the iterator has no reference or pointer members, and the loop body
changes external state.

[This matches clang bug https://llvm.org/bugs/show_bug.cgi?id=23416)
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Subject: [Bug libstdc++/66018] opendir configure test not working when GCC_NO_EXECUTABLES
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf018

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2015-05-05
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
     Ever confirmed|0                           |1

[Bug c++/66028] false positive, unused loop variable

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66028

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-05-06
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug c++/66028] false positive, unused loop variable

[ncm at cantrip dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66028

--- Comment #2 from ncm at cantrip dot org ---
This bug appears not to manifest in g++-10.2.

[Bug c++/66028] false positive, unused loop variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66028

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to ncm from comment #2)
> This bug appears not to manifest in g++-10.2.

The warning is still there for me. Maybe you forgot to test with -Wall (or
-Wunused).