[Bug c++/63604] New: [C++11] A direct-initialization of a reference should use explicit conversion functions

["kariya_mitsuru at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63604

            Bug ID: 63604
           Summary: [C++11] A direct-initialization of a reference should
                    use explicit conversion functions
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kariya_mitsuru at hotmail dot com

The sample code below should be compiled successfully but it causes compilation
error by gcc.

==========================================
struct T {};

struct S {
    explicit operator T() { return T(); }
};

int main()
{
    S s;
    T&& t(s);
    (void) t;
}
==========================================
cf. http://melpon.org/wandbox/permlink/LHgajpAXzqTbpYDc

An initialization of a reference in a direct-initialization context should use
an explicit conversion function that converts to a class prvalue.


The latest C++ standard (n4140) 13.3.1.6 [over.match.ref]/p.1.1 says that

The conversion functions of S and its base classes are considered.  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.
 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 same type as T or can
be converted to type T with a qualification conversion (4.4), are also
candidate functions.


I think that this sample code corresponds to the case “For
direct-initialization, ...”.


Note that this sample code is compiled successfully if the conversion function
returns an rvalue reference.
(cf. http://melpon.org/wandbox/permlink/kGpALX7zvzHzi7K5)

See also BUG 48453.
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From: "su at cs dot ucdavis.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/63605] New: wrong code at -O3 on x86_64-linux-gnu
Date: Mon, 20 Oct 2014 20:02:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc605

            Bug ID: 63605
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk (as well as 4.8.x and 4.9.x) miscompiles the following
code on x86_64-linux at -O3 in both 32-bit and 64-bit modes.

This is a regression from 4.7.x.

The miscompilation seems to be caused by the tree vectorizer as
-fno-tree-vectorize makes it disappear.

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/5.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 5.0.0 20141018 (experimental) [trunk revision 216429] (GCC)

$ gcc-trunk -O2 small.c; a.out
1
$ gcc-4.7 -O3 small.c; a.out
1
$
$ gcc-trunk -O3 small.c; a.out
0
$


--------------------------------


int printf (const char *, ...);

int a, b[8] = { 2, 0, 0, 0, 0, 0, 0, 0 }, c[8];

int
main ()
{
  int d;
  for (; a < 8; a++)
    {
      d = b[a] >> 1;
      c[a] = d != 0;
    }
  printf ("%d\n", c[0]);
  return 0;
}