Why does returning a reference through std::function print garbage?

I have the following simple C++20 program:

#include <iostream>
#include <string>
#include <utility>
#include <functional>

struct C {
  std::string str = {"hello"};

  const std::string& get() { return str; }
};

int main() {
    C c;
    std::function<const std::string&()> fun =
        [&]() { return c.get(); };

    std::cout << fun() << std::endl;
}

But when I compile with clang++ -std=c++20 -O0 -g t.cpp -o t and run it, I sometimes get garbage output like:

(@g

I am using clang++ 16.0.

If the lambda is returning by value, why does the compiler allow binding it to this std::function that returns by reference?
Otherwise if the lambda is returning a reference, why is it printing garbage? Returning a reference to a reference should be valid.

Solution

The lambda is indeed returning a (pr)value, not a reference.

This is warned about in the std::function documentation on cppreference:

https://en.cppreference.com/w/cpp/utility/functional/function.html

Care should be taken when a std::function, whose result type is a reference, is initialized from a lambda expression without a trailing-return-type. Due to the way auto deduction works, such lambda expression will always return a prvalue. Hence, the resulting reference will usually bind to a temporary whose lifetime ends when std::function::operator() returns.
(until C++23)

If a std::function returning a reference is initialized from a function or function object returning a prvalue (including a lambda expression without a trailing-return-type), the program is ill-formed because binding the returned reference to a temporary object is forbidden.
(since C++23)

So, you need to explicitly declare your lambda as returning a reference, eg:

[&]() -> const std::string& { return c.get(); }
//    ^^^^^^^^^^^^^^^^^^^^^