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

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

[barry.revzin at gmail dot com]

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>
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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.

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

[redi at gcc dot gnu.org]

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Barry Revzin from comment #0)
> According to [over.match.ref]: 
> 
> Those non-explicit conversion

Doesn't non-explicit rule out your conversion function?

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

[redi at gcc dot gnu.org]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-07-16
            Summary|disallowed initialization   |disallowed initialization
                   |of reference with           |of reference with explicit
                   |user-defined conversion     |user-defined conversion
                   |function                    |function
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a bug, but because of the rest of the paragraph not the part quoted
above:

  For direct-initialization, those explicit conversion functions that are not
  hidden within S and yield type “lvalue reference to cv2 T2” or “cv2 T2” or
  “rvalue reference to cv2 T2”, respectively, where T2 is the same type as T
  or can be converted to type T with a qualification conversion (4.4), are also
  candidate functions."
>From gcc-bugs-return-492538-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Thu Jul 16 12:58:25 2015
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From: "sfilippone at uniroma2 dot it" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/52846] [F2008] Support submodules
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idR846

--- Comment #11 from Salvatore Filippone <sfilippone at uniroma2 dot it> ---
Created attachment 35995
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id5995&actioníit
test case

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 63604.

*** This bug has been marked as a duplicate of bug 63604 ***