Concept `index_adjacency_list` is underconstrained

[akrzemi1]

Consider this graph representation that contains a bug in the adaptation to the Graph Container Interface:

namespace MyLibrary 
{

  struct MyEdge {
    std::string content;
    int indexOfTarget;
  
    std::string target_id(auto&&) const { return "0"; } // BUG: return type of `target_id` is different than `graph::vertex_id_t<MyLibrary>`
  };

  struct MyVertex {
    std::string content;
    std::vector<MyEdge> outEdges;
    
    std::vector<MyEdge> const& edges(auto&&) const { return outEdges; }
  };

  class MyGraph {
    std::vector<MyVertex> _vertices;
  public:
    std::vector<MyVertex> const& vertices() const { return _vertices; }
  };

  std::vector<MyEdge> const& edges(MyGraph const& g, int uid) { return g.vertices()[uid].edges(g); }
  int target(MyGraph const& g, MyEdge const& uv) { return uv.indexOfTarget; } 
}

This satisfies the concept graph::index_adjacency_list but trying to instantiate graph::dijkstra_shortest_paths ends in failure for obvious reasons.

Recommendation: The customization point target_id should have an additional constraint that its type convertible to vertex_id_t<G> (or maybe, be exactly vertex_id_t<G>).