[Bug c++/114220] New: False positive warning: possibly dangling reference to a temporary [-Wdangling-reference]

[dilyan.palauzov at aegee dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114220

            Bug ID: 114220
           Summary: False positive warning: possibly dangling reference to
                    a temporary [-Wdangling-reference]
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dilyan.palauzov at aegee dot org
  Target Milestone: ---

Created attachment 57598
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57598&action=edit
The inlined programme

This code

#include <string>
#include <unordered_map>

static const std::unordered_map<int, std::string> hashTable1 = {{2, "u"}};
static const std::unordered_map<std::string, std::string> hashTable2 = {{"u",
"u"}};

const std::string& z1(int m);
const std::string& z1(int m) { return hashTable1.at(m); }
const std::string& z2(const std::string& m);
const std::string& z2(const std::string& m) { return hashTable2.at(m); }
const std::string& z3();
const std::string& z3() { return hashTable2.at("u"); }

int main() {
  const std::string& b1 { z1(2) };
  const std::string& b2 { z2("u") };
  const std::string& b3 { z3() };
  const std::string& f1 = hashTable1.at(2);
  const std::string& f2 = hashTable2.at("u");
  printf("%s %s %s %s %s\n", b1.c_str(), b2.c_str(), f1.c_str(), f2.c_str(),
b3.c_str());
}

produces with g++ (GCC) 13.2.1 20240302:

$ g++ -Wall -Wextra -o a a.cpp
a.c: In function ‘int main()’:
a.c:16:22: warning: possibly dangling reference to a temporary
[-Wdangling-reference]
   16 |   const std::string& b2 { z2("u") };
      |                      ^~
a.c:16:29: note: the temporary was destroyed at the end of the full expression
‘z2(std::__cxx11::basic_string<char>(((const char*)"u"),
std::allocator<char>()))’
   16 |   const std::string& b2 { z2("u") };
      |                           ~~^~~~~

So std::string as first template parameter is problematic (b2), when only the
second template parameter is relevant (b3), but int as first template parameter
is fine (b1).

I think this is a bug.