C++17: More use of std::invoke().

The use of template operator() here was maybe to deal with
awkward compilers, but now we can just expect them to support
std::invoke() from C++17.

Author: murraycu

sigc++/adaptors/bind_return.h

@@ -49,8 +49,7 @@ struct bind_return_functor : public adapts<T_functor>
   template <typename... T_arg>
   inline typename unwrap_reference<T_return>::type operator()(T_arg... a)
   {
-    //TODO: Use std::invoke() here?
-    this->functor_.template operator()<type_trait_pass_t<T_arg>...>(a...);
+    std::invoke(this->functor_, a...);
     return ret_value_.invoke();
   }
 

sigc++/adaptors/exception_catch.h

@@ -93,8 +93,7 @@ struct exception_catch_functor : public adapts<T_functor>
   {
     try
     {
-      //TODO: Use std::invoke here?
-      return this->functor_.template operator()<type_trait_pass_t<T_arg>...>(a...);
+      return std::invoke(this->functor_, a...);
     }
     catch (...)
     {

sigc++/adaptors/retype.h

@@ -82,9 +82,7 @@ struct retype_functor : public adapts<T_functor>
   template <typename... T_arg>
   decltype(auto) operator()(T_arg... a)
   {
-    //TODO: Use std::invoke() here?
-    return this->functor_.template operator()<type_trait_take_t<T_type>...>(
-      static_cast<T_type>(a)...);
+    return std::invoke(this->functor_, static_cast<T_type>(a)...);
   }
 
   /** Constructs a retype_functor object that performs C-style casts on the parameters passed on to

sigc++/adaptors/retype_return.h

@@ -41,8 +41,7 @@ struct retype_return_functor : public adapts<T_functor>
   template <typename... T_arg>
   inline T_return operator()(T_arg&&... a)
   {
-    //TODO: Use std::invoke() here?
-    return T_return(this->functor_.template operator() < T_arg... > (std::forward<T_arg>(a)...));
+    return T_return(std::invoke(this->functor_, std::forward<T_arg>(a)...));
   }
 
   retype_return_functor() = default;
@@ -82,8 +81,7 @@ struct retype_return_functor<void, T_functor> : public adapts<T_functor>
   template <typename... T_arg>
   inline void operator()(T_arg... a)
   {
-    //TODO: Use std::invoke() here?
-    this->functor_.template operator()<T_arg...>(a...);
+    std::invoke(this->functor_, a...);
   }
 
   retype_return_functor() = default;

sigc++/adaptors/track_obj.h

@@ -94,9 +94,7 @@ class track_obj_functor : public adapts<T_functor>
   template <typename... T_arg>
   decltype(auto) operator()(T_arg&&... arg)
   {
-    //TODO: Use std::invoke() here?
-    return this->functor_.template operator()<type_trait_pass_t<T_arg>...>(
-      std::forward<T_arg>(arg)...);
+    return std::invoke(this->functor_, std::forward<T_arg>(arg)...);
   }
 
 #ifndef DOXYGEN_SHOULD_SKIP_THIS