Spurious warning about binding temporary to reference member in constructor

Question

I understand that if a temporary is bound to a reference member in the constructor's initializer list, the object will be destroyed as the constructor returns.

However, consider the following code:

#include <functional>
#include <iostream>

using callback_func = std::function<int(void)>;

int
func(const callback_func& callback)
{
  struct wrapper
  {
    const callback_func& w_cb;
    wrapper(const callback_func& cb) : w_cb {cb} { }
    int call() { return this->w_cb() + this->w_cb(); }
  };
  wrapper wrp {callback};
  return wrp.call();
}

int
main()
{
  std::cout << func([](){ return 21; }) << std::endl;
  return 0;
}

This looks perfectly valid to me. The callback object will live during the whole execution of the func function and no temporary copy should be made for wrapper's constructor.

Indeed, GCC 4.9.0 compiles fine with all warnings enabled.

However, GCC 4.8.2 compiler gives me the following warning:

$ g++ -std=c++11 -W main.cpp 
main.cpp: In constructor ‘func(const callback_func&)::wrapper::wrapper(const callback_func&)’:
main.cpp:12:48: warning: a temporary bound to ‘func(const callback_func&)::wrapper::w_cb’ only persists until the constructor exits [-Wextra]
     wrapper(const callback_func& cb) : w_cb {cb} { }
                                            ^

Is this a false positive or am I misunderstanding the object lifetimes?

Here are my exact compiler versions tested:

$ g++ --version
g++ (GCC) 4.8.2
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ --version
g++ (GCC) 4.9.0 20140604 (prerelease)
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 1

This is a bug in gcc 4.8 that has been fixed in 4.9. Here is the bug report:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50025

Answer 2

As pointed out by Howard Hinnant and already indicated by R Sahu's comment, this is a bug (which used to be required by the then-broken standard; thanks to Tony D for pointing this out) in the way GCC 4.8 is treating initializer lists.

Changing the constructor in my original example from

wrapper(const callback_func& cb) : w_cb {cb} { }

to

wrapper(const callback_func& cb) : w_cb (cb) { }

makes the warning with GCC 4.8.3 go away and the created executable Valgrind clean. The diff of the two assembly files is huge so I don't post it here. GCC 4.9.0 creates identical assembly code for both versions.

Next, I replaced the std::function with a user-defined struct and deleted copy and move constructors and assignment operators. Indeed, with GCC 4.8.3, this retains the warning but now also gives a (slightly more helpful) error that the above line of code calls the deleted copy constructor of the struct. As expected, there is no difference with GCC 4.9.0.