How to use source_location in a variadic template function?

Question

The C++20 feature std::source_location is used to capture information about the context in which a function is called. When I try to use it with a variadic tem

Answer 1

template 
void debug(Args&&... args,
           const std::source_location& loc = std::source_location::current());

"works", but requires to specify template arguments as there are non deducible as there are not last:

debug(42);

Demo

Possible (not perfect) alternatives include:

• use overloads with hard coded limit (old possible way to "handle" variadic):

  // 0 arguments
  void debug(const std::source_location& loc = std::source_location::current());
  
  // 1 argument
  template 
  void debug(T0&& t0,
             const std::source_location& loc = std::source_location::current());
  
  // 2 arguments
  template 
  void debug(T0&& t0, T1&& t1,
             const std::source_location& loc = std::source_location::current());
  
  // ...
  

  Demo

• to put source_location at first position, without default:

  template 
  void debug(const std::source_location& loc, Args&&... args);
  

  and

  debug(std::source_location::current(), 42);
  

  Demo

• similarly to overloads, but just use tuple as group

  template 
  void debug(Tuple&& t,
             const std::source_location& loc = std::source_location::current());
  

  or

  template 
  void debug(const std::tuple& t,
             const std::source_location& loc = std::source_location::current());
  

  with usage

  debug(std::make_tuple(42));
  

  Demo