[Bug c++/109538] New: Spurious -Werror=dangling-reference false positive

[mrsam--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109538

            Bug ID: 109538
           Summary: Spurious -Werror=dangling-reference false positive
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mr...@courier-mta.com
  Target Milestone: ---

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.