PR libstdc++/83306 make filesystem_error no-throw copyable

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index e3938d06d59..0eee684a2f6 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -43,6 +43,8 @@ 
 #include <system_error>
 #include <bits/stl_algobase.h>
 #include <bits/locale_conv.h>
+#include <ext/concurrence.h>
+#include <bits/shared_ptr.h>
 
 #if defined(_WIN32) && !defined(__CYGWIN__)
 # define _GLIBCXX_FILESYSTEM_IS_WINDOWS 1
@@ -575,30 +577,29 @@  _GLIBCXX_BEGIN_NAMESPACE_CXX11
   class filesystem_error : public std::system_error
   {
   public:
-    filesystem_error(const string& __what_arg, error_code __ec)
-    : system_error(__ec, __what_arg) { }
+    filesystem_error(const string& __what_arg, error_code __ec);
 
     filesystem_error(const string& __what_arg, const path& __p1,
-		     error_code __ec)
-    : system_error(__ec, __what_arg), _M_path1(__p1) { }
+		     error_code __ec);
 
     filesystem_error(const string& __what_arg, const path& __p1,
-		     const path& __p2, error_code __ec)
-    : system_error(__ec, __what_arg), _M_path1(__p1), _M_path2(__p2)
-    { }
+		     const path& __p2, error_code __ec);
+
+    filesystem_error(const filesystem_error&) = default;
+    filesystem_error& operator=(const filesystem_error&) = default;
+
+    // No move constructor or assignment operator.
+    // Copy rvalues instead, so that _M_impl is not left empty.
 
     ~filesystem_error();
 
-    const path& path1() const noexcept { return _M_path1; }
-    const path& path2() const noexcept { return _M_path2; }
-    const char* what() const noexcept { return _M_what.c_str(); }
+    const path& path1() const noexcept;
+    const path& path2() const noexcept;
+    const char* what() const noexcept;
 
   private:
-    std::string _M_gen_what();
-
-    path _M_path1;
-    path _M_path2;
-    std::string _M_what = _M_gen_what();
+    struct _Impl;
+    std::__shared_ptr<const _Impl> _M_impl;
   };
 
   struct path::_Cmpt : path
@@ -1158,6 +1159,8 @@  _GLIBCXX_BEGIN_NAMESPACE_CXX11
 _GLIBCXX_END_NAMESPACE_CXX11
 } // namespace filesystem
 
+extern template class __shared_ptr<const filesystem::filesystem_error::_Impl>;
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 
diff --git a/libstdc++-v3/src/filesystem/std-path.cc b/libstdc++-v3/src/filesystem/std-path.cc
index f382eb3759a..fb8898d9040 100644
--- a/libstdc++-v3/src/filesystem/std-path.cc
+++ b/libstdc++-v3/src/filesystem/std-path.cc
@@ -34,8 +34,6 @@ 
 namespace fs = std::filesystem;
 using fs::path;
 
-fs::filesystem_error::~filesystem_error() = default;
-
 constexpr path::value_type path::preferred_separator;
 
 #ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
@@ -741,47 +739,83 @@  fs::hash_value(const path& p) noexcept
   return seed;
 }
 
-namespace std
+struct fs::filesystem_error::_Impl
 {
-_GLIBCXX_BEGIN_NAMESPACE_VERSION
-namespace filesystem
-{
-  string
-  fs_err_concat(const string& __what, const string& __path1,
-		  const string& __path2)
+  _Impl(const string& what_arg, const path& p1, const path& p2)
+  : path1(p1), path2(p2), what(make_what(what_arg, &p1, &p2))
+  { }
+
+  _Impl(const string& what_arg, const path& p1)
+  : path1(p1), path2(), what(make_what(what_arg, &p1, nullptr))
+  { }
+
+  _Impl(const string& what_arg)
+  : what(make_what(what_arg, nullptr, nullptr))
+  { }
+
+  static std::string
+  make_what(const std::string& s, const path* p1, const path* p2)
   {
-    const size_t __len = 18 + __what.length()
-      + (__path1.length() ? __path1.length() + 3 : 0)
-      + (__path2.length() ? __path2.length() + 3 : 0);
-    string __ret;
-    __ret.reserve(__len);
-    __ret = "filesystem error: ";
-    __ret += __what;
-    if (!__path1.empty())
+    const std::string pstr1 = p1 ? p1->u8string() : std::string{};
+    const std::string pstr2 = p2 ? p2->u8string() : std::string{};
+    const size_t len = 18 + s.length()
+      + (pstr1.length() ? pstr1.length() + 3 : 0)
+      + (pstr2.length() ? pstr2.length() + 3 : 0);
+    std::string w;
+    w.reserve(len);
+    w = "filesystem error: ";
+    w += s;
+    if (p1)
       {
-	__ret += " [";
-	__ret += __path1;
-	__ret += ']';
+	w += " [";
+	w += pstr1;
+	w += ']';
+	if (p2)
+	  {
+	    w += " [";
+	    w += pstr2;
+	    w += ']';
+	  }
       }
-    if (!__path2.empty())
-      {
-	__ret += " [";
-	__ret += __path2;
-	__ret += ']';
-      }
-    return __ret;
+    return w;
   }
 
-_GLIBCXX_BEGIN_NAMESPACE_CXX11
+  path path1;
+  path path2;
+  std::string what;
+};
 
-  std::string filesystem_error::_M_gen_what()
-  {
-    return fs_err_concat(system_error::what(), _M_path1.u8string(),
-			 _M_path2.u8string());
-  }
+template class std::__shared_ptr<const fs::filesystem_error::_Impl>;
 
-_GLIBCXX_END_NAMESPACE_CXX11
+fs::filesystem_error::
+filesystem_error(const string& what_arg, error_code ec)
+: system_error(ec, what_arg),
+  _M_impl(std::__make_shared<_Impl>(what_arg))
+{ }
 
-} // filesystem
-_GLIBCXX_END_NAMESPACE_VERSION
-} // std
+fs::filesystem_error::
+filesystem_error(const string& what_arg, const path& p1, error_code ec)
+: system_error(ec, what_arg),
+  _M_impl(std::__make_shared<_Impl>(what_arg, p1))
+{ }
+
+fs::filesystem_error::
+filesystem_error(const string& what_arg, const path& p1, const path& p2,
+		 error_code ec)
+: system_error(ec, what_arg),
+  _M_impl(std::__make_shared<_Impl>(what_arg, p1, p2))
+{ }
+
+fs::filesystem_error::~filesystem_error() = default;
+
+const fs::path&
+fs::filesystem_error::path1() const noexcept
+{ return _M_impl->path1; }
+
+const fs::path&
+fs::filesystem_error::path2() const noexcept
+{ return _M_impl->path2; }
+
+const char*
+fs::filesystem_error::what() const noexcept
+{ return _M_impl->what.c_str(); }
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/cons.cc b/libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/cons.cc
new file mode 100644
index 00000000000..ddaaf44d1a5
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/cons.cc
@@ -0,0 +1,93 @@ 
+// 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 -lstdc++fs" }
+// { dg-do run { target c++17 } }
+// { dg-require-filesystem-ts "" }
+
+#include <filesystem>
+#include <testsuite_hooks.h>
+
+using std::filesystem::filesystem_error;
+
+bool contains(std::string_view what_str, std::string_view expected)
+{
+  return what_str.find(expected) != std::string_view::npos;
+}
+
+void
+test01()
+{
+  const char* const str = "error test";
+  const std::error_code ec = make_error_code(std::errc::is_a_directory);
+  const std::filesystem::path p1 = "test/path/one";
+  const std::filesystem::path p2 = "/test/path/two";
+
+  const filesystem_error e1(str, ec);
+  VERIFY( contains(e1.what(), str) );
+  VERIFY( !contains(e1.what(), "[]") ); // no "empty path" in the string
+  VERIFY( e1.path1().empty() );
+  VERIFY( e1.path2().empty() );
+  VERIFY( e1.code() == ec );
+
+  const filesystem_error e2(str, p1, ec);
+  VERIFY( e2.path1() == p1 );
+  VERIFY( e2.path2().empty() );
+  VERIFY( contains(e2.what(), str) );
+  VERIFY( contains(e2.what(), p1.string()) );
+  VERIFY( !contains(e2.what(), "[]") );
+  VERIFY( e2.code() == ec );
+
+  const filesystem_error e3(str, std::filesystem::path{}, ec);
+  VERIFY( e3.path1().empty() );
+  VERIFY( e3.path2().empty() );
+  VERIFY( contains(e3.what(), str) );
+  VERIFY( contains(e3.what(), "[]") );
+  VERIFY( !contains(e3.what(), "[] []") );
+  VERIFY( e3.code() == ec );
+
+  const filesystem_error e4(str, p1, p2, ec);
+  VERIFY( e4.path1() == p1 );
+  VERIFY( e4.path2() == p2 );
+  VERIFY( contains(e4.what(), str) );
+  VERIFY( contains(e4.what(), p1.string()) );
+  VERIFY( contains(e4.what(), p2.string()) );
+  VERIFY( !contains(e4.what(), "[]") );
+  VERIFY( e4.code() == ec );
+
+  const filesystem_error e5(str, p1, std::filesystem::path{}, ec);
+  VERIFY( e5.path1() == p1 );
+  VERIFY( e5.path2().empty() );
+  VERIFY( contains(e5.what(), str) );
+  VERIFY( contains(e5.what(), p1.string()) );
+  VERIFY( contains(e5.what(), "[]") );
+  VERIFY( e5.code() == ec );
+
+  const filesystem_error e6(str, std::filesystem::path{}, p2, ec);
+  VERIFY( e6.path1().empty() );
+  VERIFY( e6.path2() == p2 );
+  VERIFY( contains(e6.what(), str) );
+  VERIFY( contains(e6.what(), "[]") );
+  VERIFY( contains(e6.what(), p2.string()) );
+  VERIFY( e6.code() == ec );
+}
+
+int
+main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/copy.cc b/libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/copy.cc
new file mode 100644
index 00000000000..f085f9d6794
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/filesystem_error/copy.cc
@@ -0,0 +1,111 @@ 
+// 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 -lstdc++fs" }
+// { dg-do run { target c++17 } }
+// { dg-require-filesystem-ts "" }
+
+#include <filesystem>
+#include <testsuite_hooks.h>
+
+using std::filesystem::filesystem_error;
+
+// PR libstdc++/83306
+static_assert(std::is_nothrow_copy_constructible_v<filesystem_error>);
+static_assert(std::is_nothrow_copy_assignable_v<filesystem_error>);
+
+void
+test01()
+{
+  const char* const str = "error test";
+  const std::error_code ec = make_error_code(std::errc::is_a_directory);
+  const filesystem_error e1(str, ec);
+  auto e2 = e1;
+  VERIFY( e2.path1().empty() );
+  VERIFY( e2.path2().empty() );
+  VERIFY( std::string_view(e2.what()).find(str) != std::string_view::npos );
+  VERIFY( e2.code() == ec );
+
+  const filesystem_error e3(str, "test/path/one", ec);
+  auto e4 = e3;
+  VERIFY( e4.path1() == "test/path/one" );
+  VERIFY( e4.path2().empty() );
+  VERIFY( std::string_view(e4.what()).find(str) != std::string_view::npos );
+  VERIFY( e2.code() == ec );
+
+  const filesystem_error e5(str, "test/path/one", "/test/path/two", ec);
+  auto e6 = e5;
+  VERIFY( e6.path1() == "test/path/one" );
+  VERIFY( e6.path2() == "/test/path/two" );
+  VERIFY( std::string_view(e6.what()).find(str) != std::string_view::npos );
+  VERIFY( e2.code() == ec );
+}
+
+void
+test02()
+{
+  const char* const str = "error test";
+  const std::error_code ec = make_error_code(std::errc::is_a_directory);
+  const filesystem_error e1(str, ec);
+  filesystem_error e2("", {});
+  e2 = e1;
+  VERIFY( e2.path1().empty() );
+  VERIFY( e2.path2().empty() );
+  VERIFY( std::string_view(e2.what()).find(str) != std::string_view::npos );
+  VERIFY( e2.code() == ec );
+
+  const filesystem_error e3(str, "test/path/one", ec);
+  filesystem_error e4("", {});
+  e4 = e3;
+  VERIFY( e4.path1() == "test/path/one" );
+  VERIFY( e4.path2().empty() );
+  VERIFY( std::string_view(e4.what()).find(str) != std::string_view::npos );
+  VERIFY( e2.code() == ec );
+
+  const filesystem_error e5(str, "test/path/one", "/test/path/two", ec);
+  filesystem_error e6("", {});
+  e6 = e5;
+  VERIFY( e6.path1() == "test/path/one" );
+  VERIFY( e6.path2() == "/test/path/two" );
+  VERIFY( std::string_view(e6.what()).find(str) != std::string_view::npos );
+  VERIFY( e2.code() == ec );
+}
+
+void
+test03()
+{
+  filesystem_error e("test", std::error_code());
+  VERIFY( e.path1().empty() );
+  VERIFY( e.path2().empty() );
+  auto e2 = std::move(e);
+  // Observers must still be usable on moved-from object:
+  VERIFY( e.path1().empty() );
+  VERIFY( e.path2().empty() );
+  VERIFY( e.what() != nullptr );
+  e2 = std::move(e);
+  VERIFY( e.path1().empty() );
+  VERIFY( e.path2().empty() );
+  VERIFY( e.what() != nullptr );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+}