PR libstdc++/79162 Fix std::string regression due to LWG 2946

	PR libstdc++/79162
	* include/bits/basic_string.h (basic_string::_If_sv): Remove from the
	overload set when the argument is derived from basic_string.
	* testsuite/21_strings/basic_string/cons/char/moveable2_c++17.cc: New
	test.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2_c++17.cc:
	New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@253024 138bc75d-0d04-0410-961f-82ee72b054a4

Author: redi

libstdc++-v3/ChangeLog

@@ -1,5 +1,13 @@
 2017-09-20  Jonathan Wakely  <[email protected]>
 
+	PR libstdc++/79162
+	* include/bits/basic_string.h (basic_string::_If_sv): Remove from the
+	overload set when the argument is derived from basic_string.
+	* testsuite/21_strings/basic_string/cons/char/moveable2_c++17.cc: New
+	test.
+	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2_c++17.cc:
+	New test.
+
 	* testsuite/24_iterators/range_access_cpp17.cc: Fix order of dg-do
 	and dg-options directives. Fix invalid test.
 

libstdc++-v3/include/bits/basic_string.h

@@ -115,6 +115,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       template<typename _Tp, typename _Res>
 	using _If_sv = enable_if_t<
 	  __and_<is_convertible<const _Tp&, __sv_type>,
+		 __not_<is_convertible<const _Tp*, const basic_string*>>,
 		 __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
 	  _Res>;
 

libstdc++-v3/testsuite/21_strings/basic_string/cons/char/moveable2_c++17.cc

@@ -0,0 +1,53 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++17 } }
+
+// Copyright (C) 2011-2017 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/>.
+
+// NOTE: This makes use of the fact that we know how moveable
+// is implemented on string (via swap). If the implementation changed
+// this test may begin to fail.
+
+#include <string>
+#include <utility>
+#include <testsuite_hooks.h>
+
+class tstring : public std::basic_string<char>
+{
+public:
+  tstring() : std::basic_string<char>() {}
+  tstring(tstring&& s) : std::basic_string<char>(std::move(s)) {}
+  tstring& operator=(tstring&& s) = default;
+};
+
+void test01()
+{
+  tstring a, b;
+  a.push_back('1');
+  b = std::move(a);
+  VERIFY( b.size() == 1 && b[0] == '1' && a.size() == 0 );
+
+  tstring c(std::move(b));
+  VERIFY( c.size() == 1 && c[0] == '1' );
+  VERIFY( b.size() == 0 );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}

libstdc++-v3/testsuite/21_strings/basic_string/cons/wchar_t/moveable2_c++17.cc

@@ -0,0 +1,53 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++17 } }
+
+// Copyright (C) 2011-2017 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/>.
+
+// NOTE: This makes use of the fact that we know how moveable
+// is implemented on string (via swap). If the implementation changed
+// this test may begin to fail.
+
+#include <string>
+#include <utility>
+#include <testsuite_hooks.h>
+
+class tstring : public std::basic_string<wchar_t>
+{
+public:
+  tstring() : std::basic_string<wchar_t>() {}
+  tstring(tstring&& s) : std::basic_string<wchar_t>(std::move(s)) {}
+  tstring& operator=(tstring&& s) = default;
+};
+
+void test01()
+{
+  tstring a, b;
+  a.push_back(L'1');
+  b = std::move(a);
+  VERIFY( b.size() == 1 && b[0] == L'1' && a.size() == 0 );
+
+  tstring c(std::move(b));
+  VERIFY( c.size() == 1 && c[0] == L'1' );
+  VERIFY( b.size() == 0 );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}