[Bug c++/66893] New: disallowed initialization of reference with user-defined conversion function

["barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66893

            Bug ID: 66893
           Summary: disallowed initialization of reference with
                    user-defined conversion function
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider the following:

struct B {};
struct S { explicit operator B(); };

int main() {
    B const &t(S{});
}

According to [over.match.ref]: 

Those non-explicit conversion
functions that are not hidden within S and yield type “lvalue reference to cv2
T2” (when initializing an lvalue reference or an rvalue reference to function)
or “cv2 T2” or “rvalue reference to cv2 T2” (when initializing an rvalue
reference or an lvalue reference to function), where “cv1 T” is
reference-compatible (8.5.3) with “cv2 T2”, are candidate functions.

In this case, B const (cv1 T) is reference-compatible with B (cv2 T2), so the
operator should be a candidate function. However, gcc rejects it:

main.cpp: In function 'int main()':
main.cpp:5:19: error: invalid initialization of reference of type 'const B&'
from expression of type 'S'
     B const &t(S{});
                   ^
>From gcc-bugs-return-492394-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Thu Jul 16 04:35:53 2015
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From: "bin.x.fan at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/66842] libatomic uses multiple locks for locked atomics
Date: Thu, 16 Jul 2015 04:35:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf842

Bin Fan <bin.x.fan at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |c++

--- Comment #4 from Bin Fan <bin.x.fan at oracle dot com> ---
Since I don't see any response from C so far, I change the example to C++ code,
and change the category to c++. Could C++ folks take a look?

-bash-4.2$ cat c++11_locked_atomics.cpp
#include <atomic>
using namespace std;

#ifndef SIZE
#define SIZE 1024
#endif

typedef struct {
    char a[SIZE];
} lock_obj_t;

extern "C" {
extern void display_nlocks ();
}

int main()
{
    lock_obj_t v2 = {0};
    atomic<lock_obj_t> v1 = ATOMIC_VAR_INIT(v2);
    v2 = atomic_load (&v1);
    display_nlocks ();
    return 0;
}

gcc -shared -ldl -fPIC libmythread.c -o libmythread.so -g
g++ -std=c++11 -latomic c++11_locked_atomics.cpp -DSIZE 48 -g -L./
-Wl,-rpath=./ -lmythread
+ LD_PRELOAD=./libmythread.so
+ a.out
pthread_mutex_lock is called 32 times

The g++ version is still 4.9.2.