Expected warning maybe-uninitialized does not appear using g++13.2.0?

[Eric Batchelor <eric@bookmanager.com>]

Hello, I unintentionally stumbled upon some strange behaviour that 
occurred due to a typo.
I reproduced the behaviour where an object (std::string in my case) can 
be passed to a function by reference, uninitialized, WITHOUT a compiler 
warning.
Changing the code to pass the object by value DOES emit the warning.
I don't think the compiled code is incorrect, it segfaults presumably 
due to uninitialized members.
I understand there may seldom be a reason to use uninitialized objects, 
so "don't do that," but as I said this was unintentional and it seems 
that it should have generated a warning, which have saved some 
head-scratching.

Code to reproduce:

#include <string>
std::string f(std::string &s) {
   s.append("x");
   return s;
}
int main() {
   std::string a = f(a);
}

Compile and run (no warning):

$ g++ -o uninit_obj uninit_obj.cpp -std=c++23 -Wall -Wpedantic -Wextra 
&& ./uninit_obj
Segmentation fault (core dumped)

No difference whether using -O0 (or 1 2 3)

If I change the function to pass by value, std::string f(std::string s), 
and rerun, I get the expected compiler warning:

$ g++ -o uninit_obj uninit_obj.cpp -std=c++23 -Wall -Wpedantic -Wextra 
&& ./uninit_obj
uninit_obj.cpp: In function 'int main()':
uninit_obj.cpp:7:22: warning: 'a' may be used uninitialized 
[-Wmaybe-uninitialized]
     7 |   std::string a = f(a);
[...]
terminate called after throwing an instance of 'std::bad_alloc'
   what():  std::bad_alloc
Aborted (core dumped)

Output from g++ -v:

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/gcc13/libexec/gcc/x86_64-pc-linux-gnu/13.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-13.2.0/configure --disable-multilib 
--enable-languages=c,c++ --prefix=/usr/local/gcc13 --program-suffix=-13 
--enable-libstdcxx-backtrace=yes
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.2.0 (GCC)

Thanks