[Bug c++/48453] New: [C++0x] Invalid reference initialization via explicit conversion

[Bug c++/48453] New: [C++0x] Invalid reference initialization via explicit conversion

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

           Summary: [C++0x] Invalid reference initialization via explicit
                    conversion
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: daniel.kruegler@googlemail.com
                CC: jason@redhat.com


GCC 4.7.0 accepts the following code in C++0x mode:

template<class T>
T&& create();

template<class T, class Arg>
void test() {
 T t(create<Arg>());
 (void) t;
}

template<class T>
struct To {
 explicit operator T();
};

int main()
{
 test<int&, To<int&>>(); // a Well-formed
 test<int&&, To<int&&>>(); // b Well-formed
}

This code should be ill-formed because of the *explicit* conversion to a
reference type. 

The semantics of this program is described in 8.5.3 [dcl.init.ref] p. 5
(referring to WD N3242). In case of (a) we first inter into bullet 1 ("If the
reference is an lvalue reference"). Sub-bullet 1 does not apply, then we have
to inspect sub-bullet 2:

"has a class type (i.e., T2 is a class type), where T1 is not reference-related
to T2, and can be implicitly converted to an lvalue of type “cv3 T3,” where
“cv1 T1” is reference-compatible with “cv3 T3”106 (this conversion is selected
by enumerating the applicable conversion functions (13.3.1.6) and choosing the
best one through overload resolution (13.3)),"

This *seems* to fit, except that there is no *implicit* conversion possible
here, so we cannot enter this rule. For (a) bullet 2 does not apply and we must
fail immediately.

For (b) we ignore bullet 1, but step into bullet 2 because of the
rvalue-reference. Again, sub-bullet 1 does not apply, only sub-bullet 2 comes
near:

"has a class type (i.e., T2 is a class type), where T1 is not reference-related
to T2, and can be implicitly converted to an xvalue, class prvalue, or function
lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3”,"

Again, we have *no* implicit conversion, but now we need to try the last
sub-bullet 3:

"Otherwise, a temporary of type “cv1 T1” is created and initialized from the
initializer expression using the rules for a non-reference copy-initialization
(8.5). [..]"

The semantics of a non-reference copy-initialization forbids the consideration
of the explicit conversion function, thus, we should fail again.

[Bug c++/48453] [C++0x] Invalid reference initialization via explicit conversion

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |SUSPENDED
   Last reconfirmed|                            |2011.04.06 04:08:17
                 CC|                            |jason at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Jason Merrill <jason at gcc dot gnu.org> 2011-04-06 04:08:17 UTC ---
This seems to me like a wording issue, not a compiler bug.  The use of
"implicitly" in 8.5.3 should have been adjusted when explicit conversion
operators went in; this should be reported as a core DR.

[Bug c++/48453] [C++0x] Invalid reference initialization via explicit conversion

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

--- Comment #2 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2011-04-06 06:34:20 UTC ---
(In reply to comment #1)

I agree, I just recognize that 13.3.1.6 [over.match.ref] p. 1 b. 1 is written
to support this:

"The conversion functions of S and its base classes are considered, 
except that for copy-initialization, only the non-explicit conversion 
functions are considered. Those that are not hidden within S and yield
type “lvalue reference to cv2 T2” (when 8.5.3 requires an lvalue result) 
or “cv2 T2” or “rvalue reference to cv2 T2” (when 8.5.3 requires an rvalue 
result), where “cv1 T” is reference-compatible (8.5.3) with “cv2 T2”, are 
candidate functions."

IMO the core language should strike the reference to "implicit conversions" in
8.5.3.

[Bug c++/48453] [C++0x] Invalid reference initialization via explicit conversion

[jens.maurer at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

Jens Maurer <jens.maurer at gmx dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jens.maurer at gmx dot net

--- Comment #3 from Jens Maurer <jens.maurer at gmx dot net> 2011-04-06 21:56:51 UTC ---
Agreed.  The wording in the standard should be fixed.

[Bug c++/48453] [C++0x] Invalid reference initialization via explicit conversion

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
It would be nice to have a DR # for this. In any case, if in practice the
compiler is already Ok, we can probably close it, right?

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[C++0x] Invalid reference   |[C++0x] [DR 1287] Invalid
                   |initialization via explicit |reference initialization
                   |conversion                  |via explicit conversion

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
Added DR 1287 reference.  I'm inclined to leave the bug suspended until that
issue is resolved.

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Ok, thanks.

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

--- Comment #7 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Oh, by the way, the status is <ready>.

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|SUSPENDED                   |NEW

--- Comment #8 from Paolo Carlini <paolo.carlini at oracle dot com> ---
This is now in C++14. Time to add the testcase and close the bug.

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

--- Comment #9 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Paolo Carlini from comment #8)
> This is now in C++14. Time to add the testcase and close the bug.
I agree.
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From: "paolo.carlini at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/65497] Call to member function template named the same as class template is rejected
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide497

Paolo Carlini <paolo.carlini at oracle dot com> changed:

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

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Same issue, really.

*** This bug has been marked as a duplicate of bug 61642 ***

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

--- Comment #10 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Fri Mar 20 17:06:18 2015
New Revision: 221537

URL: https://gcc.gnu.org/viewcvs?rev=221537&root=gcc&view=rev
Log:
2015-03-20  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/48453
    * g++.dg/cpp0x/pr48453.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/pr48453.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/48453] [C++0x] [DR 1287] Invalid reference initialization via explicit conversion

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48453

Paolo Carlini <paolo.carlini at oracle dot com> changed:

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

--- Comment #11 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Done.