This is where Concepts will shine

#include <string>
#include <iostream>
#include <experimental/type_traits>

namespace xp = std::experimental;

template<typename... Args>
concept bool StringArgs = xp::is_constructible_v<std::string, Args...>;

void internal_func(const std::string& s)
{
  std::cout << s << '\n';
}

StringArgs{...T}
void do_with_string(T&&... t)
{
  internal_func({std::forward<T>(t)...});
}

int main()
{
  do_with_string("foobar", 3U);
  do_with_string(std::string("bar"));
  do_with_string(3.1);
}

Output:

test.cpp: In function 'int main()':
test.cpp:25:21: error: cannot call function 'void do_with_string(T&& ...) [with T = {double}]'
   do_with_string(3.1);
                 ^
test.cpp:16:6: note:   constraints not satisfied
 void do_with_string(T&&... t)
      ^~~~~~~~~~~~~~
test.cpp:16:6: note:   concept 'StringArgs<double>' was not satisfied

Live example),filterAsm:(commentOnly:!t,directives:!t,labels:!t),version:3).