Deduce template argument from std::function call signature

Question

Consider this template function:

template
ReturnT foo(const std::function& fun)
{
    return fun();
}
         


        

Answer 1

A function pointer of type bool (*)() can be converted to std::function but is not the same type, so a conversion is needed. Before the compiler can check whether that conversion is possible it needs to deduce ReturnT as bool, but to do that it needs to already know that std::function is a possible conversion, which isn't possible until it deduces ReturnT ... see the problem?

Also, consider that bool(*)() could also be converted to std::function or std::function ... which should be deduced?

Consider this simplification:

template
  struct function
  {
    template
      function(U) { }
  };

template
  void foo(function)
  { }

int main()
{
    foo(1);
}

How can the compiler know whether you wanted to create function or function or function when they can all be constructed from an int?