tgba_digraph: Simplify declaration of forwarded methods.

* src/misc/common.hh (SPOT_RETURN): New macro.
* src/tgba/tgbagraph.hh (out, states, transitions): Use it.

src/misc/common.hh

@@ -95,4 +95,9 @@
 
 #define SPOT_UNIMPLEMENTED() throw std::runtime_error("unimplemented");
 
+
+// Useful when forwarding methods such as:
+//   auto func(int param) SPOT_RETURN(implem_.func(param));
+#define SPOT_RETURN(code) -> decltype(code) { return code; }
+
 #endif // SPOT_MISC_COMMON_HH

src/tgba/tgbagraph.hh

@@ -332,17 +332,20 @@ namespace spot
       return g_.new_transition(src, dst, cond, acc);
     }
 
-    internal::state_out<const graph_t>
-    out(unsigned src) const
-    {
-      return g_.out(src);
-    }
-
-    internal::state_out<graph_t>
-    out(unsigned src)
-    {
-      return g_.out(src);
-    }
+    auto out(unsigned src) const
+      SPOT_RETURN(g_.out(src));
+    auto out(unsigned src)
+      SPOT_RETURN(g_.out(src));
+
+    auto states() const
+      SPOT_RETURN(g_.states());
+    auto states()
+      SPOT_RETURN(g_.states());
+
+    auto transitions() const
+      SPOT_RETURN(g_.transitions());
+    auto transitions()
+      SPOT_RETURN(g_.transitions());
 
     /// \brief Copy the acceptance conditions of another tgba.
     void copy_acceptance_conditions_of(const tgba *a)