PR libstdc++/79162 Fix std::string regression due to LWG 2946 (old ABI)

	PR libstdc++/79162
	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
	(basic_string::_If_sv): Remove from the overload set when the
	argument is derived from basic_string.

From-SVN: r253035

libstdc++-v3/ChangeLog

 2017-09-20  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/79162
+	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
+	(basic_string::_If_sv): Remove from the overload set when the
+	argument is derived from basic_string.
+
+	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

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

@@ -3439,6 +3439,7 @@ _GLIBCXX_END_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>;