Using 'void' template arguments in C++

Question

Take the following minimal example:

using Type1 = std::function;

template 
using Type2 = std::function;

         


        

Answer 1

Several answers already explain the rationale. To add to those answers, the specification says (C++11 §8.3.5[dcl.func]/4):

A parameter list consisting of a single unnamed parameter of non-dependent type void is equivalent to an empty parameter list. Except for this special case, a parameter shall not have type cv void.

In your Type2 example, the T in void(T) is a dependent type--it depends on a template parameter.