[PATCH] PR libstdc++/85818 ensure path::preferred_separator is defined

[Jonathan Wakely]

Because path.cc is compiled with -std=gnu++17 the static constexpr
data member is implicitly 'inline' and so no definition gets emitted
unless it gets used in that translation unit. Other translation units
built as C++11 or C++14 still require a namespace-scope definition of
the variable, so mark the definition as used.

        PR libstdc++/85818
        * src/filesystem/path.cc (path::preferred_separator): Add used
        attribute.
        * testsuite/experimental/filesystem/path/preferred_separator.cc: New.

Tested powerpc64le-linux, committed to trunk. Backport to gcc-8 to
follow.




commit e80e2abad41faf6bd62bd1f08baa86f71714811e
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu May 17 13:59:00 2018 +0100

    PR libstdc++/85818 ensure path::preferred_separator is defined
    
    Because path.cc is compiled with -std=gnu++17 the static constexpr
    data member is implicitly 'inline' and so no definition gets emitted
    unless it gets used in that translation unit. Other translation units
    built as C++11 or C++14 still require a namespace-scope definition of
    the variable, so mark the definition as used.
    
            PR libstdc++/85818
            * src/filesystem/path.cc (path::preferred_separator): Add used
            attribute.
            * testsuite/experimental/filesystem/path/preferred_separator.cc: 
New.

diff --git a/libstdc++-v3/src/filesystem/path.cc 
b/libstdc++-v3/src/filesystem/path.cc
index 4d84168d742..899d94e0067 100644
--- a/libstdc++-v3/src/filesystem/path.cc
+++ b/libstdc++-v3/src/filesystem/path.cc
@@ -33,7 +33,7 @@ using fs::path;
 
 fs::filesystem_error::~filesystem_error() = default;
 
-constexpr path::value_type path::preferred_separator;
+constexpr path::value_type path::preferred_separator [[gnu::used]];
 
 path&
 path::remove_filename()
diff --git 
a/libstdc++-v3/testsuite/experimental/filesystem/path/preferred_separator.cc 
b/libstdc++-v3/testsuite/experimental/filesystem/path/preferred_separator.cc
new file mode 100644
index 00000000000..b470e312bb1
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/filesystem/path/preferred_separator.cc
@@ -0,0 +1,34 @@
+// 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 "-O0 -lstdc++fs -std=gnu++11" }
+// { dg-do link { target c++11 } }
+
+#include <experimental/filesystem>
+
+std::experimental::filesystem::path::value_type
+test01()
+{
+  auto* sep = &std::experimental::filesystem::path::preferred_separator;
+  return *sep;
+}
+
+int
+main()
+{
+  test01();
+}