false positive, unused loop variable

[Quuxplusone]

Bugzilla Link	PR23416
Status	CONFIRMED
Importance	P normal
Reported by	ncm (ncm@cantrip.org)
Reported on	2015-05-05 12:01:37 -0700
Last modified on	2021-03-20 17:47:34 -0700
Version	unspecified
Hardware	PC Linux
CC	davidfromonline@gmail.com, dgregor@apple.com, llvm-bugs@lists.llvm.org
Fixed by commit(s)
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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: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.

[Quuxplusone]

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

[Quuxplusone]

See it live: https://godbolt.org/z/q8czzh

The workaround since C++17 is to modify your for loop to be

for ([[maybe_unused]] int i : range{0,10})

but I agree this warning is annoying when trying to use an integer range.