LWG 2989 hide path iostream operators from normal lookup

By only defining these operators as friends (with no namespace-scope
declaration) they can only be found by ADL and do not participate in
overload resolution for arguments of types other than path.

	LWG 2989 hide path iostream operators from normal lookup
	* include/bits/fs_path.h (operator<<, operator>>): Define inline as
	friends.
	* testsuite/27_io/filesystem/path/io/dr2989.cc: New.

From-SVN: r261711

libstdc++-v3/ChangeLog

 2018-06-18  Jonathan Wakely  <jwakely@redhat.com>
 
+	LWG 2989 hide path iostream operators from normal lookup
+	* include/bits/fs_path.h (operator<<, operator>>): Define inline as
+	friends.
+	* testsuite/27_io/filesystem/path/io/dr2989.cc: New.
+
 	LWG 3050 Fix cv-qualification of convertibility constraints
 	* include/std/chrono (duration, operator*, operator/, operator%): Use
 	const-qualified type as source type in is_convertible constraints.

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

@@ -363,6 +363,26 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
     iterator begin() const;
     iterator end() const;
 
+    /// Write a path to a stream
+    template<typename _CharT, typename _Traits>
+      friend std::basic_ostream<_CharT, _Traits>&
+      operator<<(std::basic_ostream<_CharT, _Traits>& __os, const path& __p)
+      {
+	__os << std::quoted(__p.string<_CharT, _Traits>());
+	return __os;
+      }
+
+    /// Read a path from a stream
+    template<typename _CharT, typename _Traits>
+      friend std::basic_istream<_CharT, _Traits>&
+      operator>>(std::basic_istream<_CharT, _Traits>& __is, path& __p)
+      {
+	std::basic_string<_CharT, _Traits> __tmp;
+	if (__is >> std::quoted(__tmp))
+	  __p = std::move(__tmp);
+	return __is;
+      }
+
     // Create a basic_string by reading until a null character.
     template<typename _InputIterator,
 	     typename _Traits = std::iterator_traits<_InputIterator>,
@@ -505,26 +525,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
     return __result;
   }
 
-  /// Write a path to a stream
-  template<typename _CharT, typename _Traits>
-    basic_ostream<_CharT, _Traits>&
-    operator<<(basic_ostream<_CharT, _Traits>& __os, const path& __p)
-    {
-      __os << std::quoted(__p.string<_CharT, _Traits>());
-      return __os;
-    }
-
-  /// Read a path from a stream
-  template<typename _CharT, typename _Traits>
-    basic_istream<_CharT, _Traits>&
-    operator>>(basic_istream<_CharT, _Traits>& __is, path& __p)
-    {
-      basic_string<_CharT, _Traits> __tmp;
-      if (__is >> std::quoted(__tmp))
-	__p = std::move(__tmp);
-      return __is;
-    }
-
   template<typename _InputIterator>
     inline auto
     u8path(_InputIterator __first, _InputIterator __last)

libstdc++-v3/testsuite/27_io/filesystem/path/io/dr2989.cc

+// Copyright (C) 2018 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/>.
+
+// { dg-options "-std=gnu++17" }
+// { dg-do compile { target c++17 } }
+
+#include <iostream>
+#include <filesystem>
+
+using namespace std::filesystem;
+
+struct P {
+  operator path&();
+};
+
+void foo(std::iostream& s) {
+  P p;
+  s << p; // { dg-error "no match" }
+  s >> p; // { dg-error "no match" }
+}
+// { dg-prune-output "no type .* std::enable_if" }