[Bug libstdc++/91007] New: stable_sort segfaults on -O3

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91007

            Bug ID: 91007
           Summary: stable_sort segfaults on -O3
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Reduced from StackOverflow (https://stackoverflow.com/q/56779605/2069064):

#include <vector>
#include <algorithm>

int main() {
    using V = std::vector<int>;
    std::vector<V> conns{
        V{0,2},
        V{0,2},
        V{0,2},
    };  

    std::stable_sort(conns.begin(), conns.end(),
        [](const V& a, const V& b){ 
            return a[0] < b[0];
        }
    );  
}

On gcc 9.1, compiled with -O3, this segfaults. With -O2 and under, it's fine.
On gcc 8.3, it's fine.