[Bug c++/87503] New: Spurious -Wunused-but-set-variable warning in template function

[chtz at informatik dot uni-bremen.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87503

            Bug ID: 87503
           Summary: Spurious -Wunused-but-set-variable warning in template
                    function
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chtz at informatik dot uni-bremen.de
  Target Milestone: ---

Using g++ 8.1.0 and compiling the following minimal example with -Wall

  template<class Index, bool flag>
  Index bar(Index rows, Index cols) {
    const bool left = flag; 
    Index x = left ? rows : cols;
    return x;
  }

  int main() {
    return bar<int, true>(1,2);
  }

I'm getting this:
  warning: variable ‘left’ set but not used [-Wunused-but-set-variable]

This works fine with g++-7.3 (or earlier) and seems to be fixed in 8.2.0 (I
don't have easy access to 8.1.1. If it has been fixed there already, apologies
for the noise)

Maybe related issue: Bug 85827