```
In file included from build/cythonized/sage/graphs/base/boost_graph.cpp:1278:
build/cythonized/sage/graphs/base/boost_interface.cpp:119:34: error: non-constant-expression cannot be narrowed from type 'value_type' (aka 'unsigned long') to 'int' in initializer list [-Wc++11-narrowing-const-reference]
  119 | to_return.push_back({index[boost::source(*ei, graph)],
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
build/cythonized/sage/graphs/base/boost_graph.cpp:17503:29: note: in instantiation of member function 'BoostGraph<boost::vecS, boost::vecS, boost::directedS, boost::vecS, boost::property<boost::edge_weight_t, double>>::edge_list' requested here
 17503 |   __pyx_v_edges = __pyx_v_g.edge_list();
                                     ^
```

https://clang.llvm.org/docs/DiagnosticsReference.html#wc-11-narrowing-const-reference

To silence the error assign to a v_index outside of the
initializer list.

…into conda-python-13