gcc 13.0.1 would like to register a complaint:

t.C:17:14: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
   17 |   const int &v=optional_arg_or(def, 0);
      |              ^
t.C:17:31: note: the temporary was destroyed at the end of the full expression ‘optional_arg_or(def, 0)’
   17 |   const int &v=optional_arg_or(def, 0);
      |                ~~~~~~~~~~~~~~~^~~~~~~~

for:

#include <optional>
#include <utility>

const int &optional_arg_or(std::optional<int> &def,
			   int &&def_val)
{
  def = def_val;

  return *def;
}


int gimme()
{
  std::optional<int> def;

  const int &v=optional_arg_or(def, 0);

  int bologna=v;
  
  return bologna;
}

Changing optional_arg_or's parameter to "int def_val" silences the complaint. gcc appears to be voicing an objection that optional_arg_or returns a reference to a temporary that gets created during the function call and destroyed at the conclusion of the function call. However that temporary gets placed into a std::optional, and then a reference to the std::optional gets returned.