[Bug c++/81767] New: Unused variable warning with structured binding

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81767

            Bug ID: 81767
           Summary: Unused variable warning with structured binding
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

The following code produces "warning: unused variable 'ignore'
[-Wunused-variable]":


int main() {    
    int m[2][2] = {{1,2}, {3, 4}};
    int sum = 0;
    for (auto& [ignore, value] : m) {
        sum += value;
    }

    return sum - 6;
}

This is not good, as multiple users would like to ignore part of the result and
deal only with the other part.

Seems better to warn only if result of structured binding does not used at all:

for (auto& [ignore, value] : m) { // "warning: structured binding result not
used
    sum += 0;
}

Otherwise emit no warning (just like clang does).