graph: Add a test close to the tgba_explicit_string setup.

* src/graph/graph.hh (boxed_label): If State_Data==void, inherit from
std::tuple<> and implement a data() method.
(digraph::state_data): Return by reference.
* src/graphtest/ngraph.cc, src/graphtest/ngraph.test: Test the
case where State_Data implements the spot::state interface.

src/graph/graph.hh

@@ -42,6 +42,7 @@ namespace spot
     template <typename Data, bool boxed = !std::is_class<Data>::value>
     struct boxed_label
     {
+      typedef Data data_t;
       Data label;
 
       template <typename... Args>
@@ -68,13 +69,26 @@ namespace spot
     };
 
     template <>
-    struct boxed_label<void, true>
+    struct boxed_label<void, true>: public std::tuple<>
     {
+      typedef std::tuple<> data_t;
+      std::tuple<>& data()
+      {
+	return *this;
+      }
+
+      const std::tuple<>& data() const
+      {
+	return *this;
+      }
+
     };
 
     template <typename Data>
     struct boxed_label<Data, false>: public Data
     {
+      typedef Data data_t;
+
       template <typename... Args>
       boxed_label(Args&&... args):
 	Data{std::forward<Args>(args)...}
@@ -302,15 +316,16 @@ namespace spot
       return s;
     }
 
-    // May not be called on states with no data.
-    State_Data
+    // Do not use State_Data& as return type, because State_Data might
+    // be void.
+    typename state_storage_t::data_t&
     state_data(state s)
     {
       return states_[s].data();
     }
 
     // May not be called on states with no data.
-    State_Data
+    const typename state_storage_t::data_t&
     state_data(state s) const
     {
       return states_[s].data();

src/graphtest/ngraph.cc

@@ -20,6 +20,7 @@
 
 #include <iostream>
 #include "graph/ngraph.hh"
+#include "tgba/state.hh"
 
 template <typename SL, typename TL>
 void
@@ -339,6 +340,74 @@ bool f8()
   return f == 69;
 }
 
+struct my_state: public spot::state
+{
+public:
+  virtual ~my_state() noexcept
+  {
+  }
+
+  virtual int compare(const spot::state* other) const
+  {
+    auto o = down_cast<const my_state*>(other);
+    assert(o);
+
+    // Do not simply return "other - this", it might not fit in an int.
+    if (o < this)
+      return -1;
+    if (o > this)
+      return 1;
+    return 0;
+  }
+
+  virtual size_t hash() const
+  {
+    return
+      reinterpret_cast<const char*>(this) - static_cast<const char*>(nullptr);
+  }
+
+  virtual my_state*
+  clone() const
+  {
+    return const_cast<my_state*>(this);
+  }
+
+  virtual void destroy() const
+  {
+  }
+
+  friend std::ostream& operator<<(std::ostream& os, const my_state&)
+  {
+    return os;
+  }
+};
+
+bool f9()
+{
+  typedef spot::digraph<my_state, int_pair> graph_t;
+  graph_t g(3);
+  spot::named_graph<graph_t, std::string> gg(g);
+  auto s1 = gg.new_state("s1");
+  gg.new_state("s2");
+  gg.new_state("s3");
+  gg.new_transition("s1", "s2", 1, 3);
+  gg.new_transition("s1", "s3", 2, 5);
+  gg.new_transition("s2", "s3", 3, 7);
+  gg.new_transition("s3", "s2", 4, 9);
+
+  dot(std::cout, gg);
+
+  int f = 0;
+  for (auto& t: g.out(s1))
+    {
+      f += t.one + t.two;
+    }
+
+
+  return (f == 11) &&
+    g.state_data(s1).compare(&g.state_data(gg.get_state("s1"))) == 0 &&
+    g.state_data(s1).compare(&g.state_data(gg.get_state("s2"))) != 0;
+}
 
 int main()
 {
@@ -350,6 +419,7 @@ int main()
   bool a6 = f6();
   bool a7 = f7();
   bool a8 = f8();
+  bool a9 = f9();
   std::cout << a1 << ' '
 	    << a2 << ' '
 	    << a3 << ' '
@@ -357,6 +427,7 @@ int main()
 	    << a5 << ' '
 	    << a6 << ' '
 	    << a7 << ' '
-	    << a8 << '\n';
-  return !(a1 && a2 && a3 && a4 && a5 && a6 && a7 && a8);
+	    << a8 << ' '
+	    << a9 << '\n';
+  return !(a1 && a2 && a3 && a4 && a5 && a6 && a7 && a8 && a9);
 }

src/graphtest/ngraph.test

@@ -80,7 +80,16 @@ digraph {
  2 [label="s3\n(4,8)"]
  2 -> 1 [label="(4,9)"]
 }
-1 1 1 1 1 1 1 1
+digraph {
+ 0 [label="s1\n"]
+ 0 -> 1 [label="(1,3)"]
+ 0 -> 2 [label="(2,5)"]
+ 1 [label="s2\n"]
+ 1 -> 2 [label="(3,7)"]
+ 2 [label="s3\n"]
+ 2 -> 1 [label="(4,9)"]
+}
+1 1 1 1 1 1 1 1 1
 EOF
 
 diff stdout expected