From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-413271-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 15534 invoked by alias); 25 Jan 2013 19:15:53 -0000
Received: (qmail 15201 invoked by uid 48); 25 Jan 2013 19:15:36 -0000
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/56112] New: [4.8 Regression] cannot create unordered_map from range of types convertible to value_type
Date: Fri, 25 Jan 2013 19:15:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Keywords: rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields:
Message-ID: <bug-56112-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2013-01/txt/msg02390.txt.bz2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56112

             Bug #: 56112
           Summary: [4.8 Regression] cannot create unordered_map from
                    range of types convertible to value_type
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


#include <unordered_map>

struct S
{
    operator std::pair<const int, int>() const { return {}; }
};

int main()
{
    S s[1];
    std::unordered_map<int, int> m(s, s+1);   // error

    std::pair<const int, int> p = *s;  // OK
    m.insert(s, s+1);  // OK
}


This is valid (value_type is EmplaceConstructible into the container from s[0])
but it fails to compile because of the change to use std::__detail::_Select1st
instead of std::_Select1st<value_type>

The template argument for _Select1st::operator() cannot be deduced from the
object of type S. The problem boils down to this:

#include <utility>

struct S
{
    operator std::pair<const int, int>() const { return {}; }
};

void f()
{
    S s;
    std::get<0>(s);
}

This is not valid, but the range constructors of unordered_map and
unordered_multimap assume using std::get<0> on the iterator's value type will
work, which is not a valid assumption.