[PATCH] Add deleted std::thread(const thread&&) constructor

[Jonathan Wakely]

LWG 2097 says that the templated std::thread(Callable&&, Args&&...)
constructor should not participate in overload resolution when
decay_t<Callable> is std::thread. Rather than removing it from the
overload set we just ensure that there are better matches, but we fail
to do so for const rvalues. This fixes it.

        PR libstdc++/78956
        * include/std/thread (thread(const thread&&)): Add deleted
        constructor.
        * testsuite/30_threads/thread/cons/lwg2097.cc: New test.

Tested powerpc64le-linux, committed to trunk.


commit a8b2bad2dda80054e3b8cc5a4d1c2c39226070d5
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Jan 3 12:09:59 2017 +0000

    Add deleted std::thread(const thread&&) constructor
    
        PR libstdc++/78956
        * include/std/thread (thread(const thread&&)): Add deleted
        constructor.
        * testsuite/30_threads/thread/cons/lwg2097.cc: New test.

diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index f725bdb..8e2cb68 100644
--- a/libstdc++-v3/include/std/thread
+++ b/libstdc++-v3/include/std/thread
@@ -108,6 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     // 2097.  packaged_task constructors should be constrained
     thread(thread&) = delete;
     thread(const thread&) = delete;
+    thread(const thread&&) = delete;
 
     thread(thread&& __t) noexcept
     { swap(__t); }
diff --git a/libstdc++-v3/testsuite/30_threads/thread/cons/lwg2097.cc 
b/libstdc++-v3/testsuite/30_threads/thread/cons/lwg2097.cc
new file mode 100644
index 0000000..165c649
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/thread/cons/lwg2097.cc
@@ -0,0 +1,29 @@
+// { dg-do compile { target c++11 } }
+// { dg-require-cstdint "" }
+// { dg-require-gthreads "" }
+
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <thread>
+
+using std::thread;
+using std::is_constructible;
+
+static_assert( !is_constructible<thread, thread&>::value, "" );
+static_assert( !is_constructible<thread, const thread&>::value, "" );
+static_assert( !is_constructible<thread, const thread>::value, "" );