On the various ways of getting the result type of a function

In a context where the type of the result of a function call must be deduced, C++ seems to be more that happy to help us, providing (at least to my knowledge the following) two solutions :

• The result of type trait :

  std::result_of<F(Args...)>::type
  

• A core language syntax :

  decltype(std::declval<F>()(std::declval<Args>()...); 
  

My question is, are the any differences between the two? Is there a context where one cannot be substituted by the other and if not why did we need a type trait to do something the language could do out of the box ?