The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License. Creative Commons License

// raw code

#include "wrapper.hpp"
#include <string>
#include <vector>

void printStringWrapper(Wrapper<std::string>) {
void printVectorWrapper(Wrapper<std::vector<std::string>>) {

int main()
  // implicit conversion from string literal to string:
  std::string s1{"hello"};
  std::string s2 = "hello";                       // OK
  Wrapper<std::string> ws1{"hello"};
  Wrapper<std::string> ws2 = "hello";             // OK
  printStringWrapper("hello");                    // OK

  // NO implicit conversion from size to vector:
  std::vector<std::string> v1{42u};
  std::vector<std::string> v2 = 42u;              // ERROR: explicit
  Wrapper<std::vector<std::string>> wv1{42u};
  Wrapper<std::vector<std::string>> wv2 = 4u2;    // ERROR: explicit
  printVectorWrapper(42u);                        // ERROR: explicit