Fix issue with vector<tuple<...>> in gt_hash_map/set

This also transforms a run-time exception into a static_assert, in case
there is no available default empty or deleted key of the specified type.

src/graph/hash_map_wrap.hh

@@ -31,14 +31,13 @@
 #include SPARSEHASH_INCLUDE(dense_hash_map)
 #endif
 
-template <class Key>
-struct no_empty_key : std::exception {};
-
 template <class Key>
 struct empty_key
 {
     static Key get()
     {
+        static_assert(std::is_arithmetic<Key>::value,
+                      "no default empty key for sparsehash!");
         return dispatch(typename std::is_arithmetic<Key>::type());
     }
 
@@ -49,7 +48,8 @@ struct empty_key
 
     static Key dispatch(std::integral_constant<bool, false>)
     {
-        throw no_empty_key<Key>();
+        assert(false);
+        return Key();
     }
 };
 
@@ -83,15 +83,23 @@ struct empty_key<std::tuple<Val, Vals...>>
     }
 };
 
+template <>
+struct empty_key<std::tuple<>>
+{
+    static std::tuple<> get()
+    {
+        return std::tuple<>();
+    }
+};
 
-template <class Key>
-struct no_deleted_key : std::exception {};
 
 template <class Key>
 struct deleted_key
 {
     static Key get()
     {
+        static_assert(std::is_arithmetic<Key>::value,
+                      "no default deleted key for sparsehash!");
         return dispatch(typename std::is_arithmetic<Key>::type());
     }
 
@@ -105,7 +113,8 @@ struct deleted_key
 
     static Key dispatch(std::integral_constant<bool, false>)
     {
-        throw no_deleted_key<Key>();
+        assert(false);
+        return Key();
     }
 };
 
@@ -139,6 +148,15 @@ struct deleted_key<std::tuple<Val, Vals...>>
     }
 };
 
+template <>
+struct deleted_key<std::tuple<>>
+{
+    static std::tuple<> get()
+    {
+        return std::tuple<>();
+    }
+};
+
 template<class Key,
          class Hash = std::hash<Key>,
          class Pred = std::equal_to<Key>,