PR libstdc++/83891 fix path::is_absolute() for non-POSIX targets

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index 53bf237b547..51af2891647 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -376,7 +376,7 @@  _GLIBCXX_BEGIN_NAMESPACE_CXX11
     bool has_filename() const;
     bool has_stem() const;
     bool has_extension() const;
-    bool is_absolute() const;
+    bool is_absolute() const { return has_root_directory(); }
     bool is_relative() const { return !is_absolute(); }
 
     // generation
@@ -1071,16 +1071,6 @@  _GLIBCXX_BEGIN_NAMESPACE_CXX11
     return ext.first && ext.second != string_type::npos;
   }
 
-  inline bool
-  path::is_absolute() const
-  {
-#ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
-    return has_root_name();
-#else
-    return has_root_directory();
-#endif
-  }
-
   inline path::iterator
   path::begin() const
   {
diff --git a/libstdc++-v3/include/experimental/bits/fs_path.h b/libstdc++-v3/include/experimental/bits/fs_path.h
index 3b4011e6414..ada7c1791aa 100644
--- a/libstdc++-v3/include/experimental/bits/fs_path.h
+++ b/libstdc++-v3/include/experimental/bits/fs_path.h
@@ -368,7 +368,7 @@  _GLIBCXX_BEGIN_NAMESPACE_CXX11
     bool has_filename() const;
     bool has_stem() const;
     bool has_extension() const;
-    bool is_absolute() const;
+    bool is_absolute() const { return has_root_directory(); }
     bool is_relative() const { return !is_absolute(); }
 
     // iterators
@@ -999,16 +999,6 @@  _GLIBCXX_BEGIN_NAMESPACE_CXX11
     return ext.first && ext.second != string_type::npos;
   }
 
-  inline bool
-  path::is_absolute() const
-  {
-#ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
-    return has_root_name();
-#else
-    return has_root_directory();
-#endif
-  }
-
   inline path::iterator
   path::begin() const
   {
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_absolute.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_absolute.cc
new file mode 100644
index 00000000000..6b5c098489a
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_absolute.cc
@@ -0,0 +1,62 @@ 
+// { dg-options "-std=gnu++17 -lstdc++fs" }
+// { dg-do run { target c++17 } }
+// { dg-require-filesystem-ts "" }
+
+// 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/>.
+
+// 30.11.7.4.9 path decomposition [fs.path.decompose]
+
+#include <filesystem>
+#include <testsuite_hooks.h>
+
+using std::filesystem::path;
+
+void
+test01()
+{
+  VERIFY( path("/").is_absolute() );
+  VERIFY( path("/foo").is_absolute() );
+  VERIFY( path("/foo/").is_absolute() );
+  VERIFY( path("/foo/bar").is_absolute() );
+  VERIFY( path("/foo/bar/").is_absolute() );
+  VERIFY( ! path("foo").is_absolute() );
+  VERIFY( ! path("foo/").is_absolute() );
+  VERIFY( ! path("foo/bar").is_absolute() );
+  VERIFY( ! path("foo/bar/").is_absolute() );
+  VERIFY( ! path("c:").is_absolute() );
+  VERIFY( ! path("c:foo").is_absolute() );
+  VERIFY( ! path("c:foo/").is_absolute() );
+  VERIFY( ! path("c:foo/bar").is_absolute() );
+  VERIFY( ! path("c:foo/bar/").is_absolute() );
+#ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
+  const bool drive_letter_is_root_name = true;
+#else
+  const bool drive_letter_is_root_name = false;
+#endif
+  VERIFY( path("c:/").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo/").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo/bar").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo/bar/").is_absolute() == drive_letter_is_root_name );
+}
+
+int
+main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_relative.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_relative.cc
index a647e8e831f..d8c09414965 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_relative.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/query/is_relative.cc
@@ -19,7 +19,7 @@ 
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// 8.4.9 path decomposition [path.decompose]
+// 30.11.7.4.9 path decomposition [fs.path.decompose]
 
 #include <filesystem>
 #include <testsuite_hooks.h>
diff --git a/libstdc++-v3/testsuite/experimental/filesystem/path/query/is_absolute.cc b/libstdc++-v3/testsuite/experimental/filesystem/path/query/is_absolute.cc
new file mode 100644
index 00000000000..1b99eaceef4
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/filesystem/path/query/is_absolute.cc
@@ -0,0 +1,62 @@ 
+// { dg-options "-std=gnu++17 -lstdc++fs" }
+// { dg-do run { target c++17 } }
+// { dg-require-filesystem-ts "" }
+
+// 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/>.
+
+// 8.4.9 path decomposition [path.decompose]
+
+#include <filesystem>
+#include <testsuite_hooks.h>
+
+using std::filesystem::path;
+
+void
+test01()
+{
+  VERIFY( path("/").is_absolute() );
+  VERIFY( path("/foo").is_absolute() );
+  VERIFY( path("/foo/").is_absolute() );
+  VERIFY( path("/foo/bar").is_absolute() );
+  VERIFY( path("/foo/bar/").is_absolute() );
+  VERIFY( ! path("foo").is_absolute() );
+  VERIFY( ! path("foo/").is_absolute() );
+  VERIFY( ! path("foo/bar").is_absolute() );
+  VERIFY( ! path("foo/bar/").is_absolute() );
+  VERIFY( ! path("c:").is_absolute() );
+  VERIFY( ! path("c:foo").is_absolute() );
+  VERIFY( ! path("c:foo/").is_absolute() );
+  VERIFY( ! path("c:foo/bar").is_absolute() );
+  VERIFY( ! path("c:foo/bar/").is_absolute() );
+#ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
+  const bool drive_letter_is_root_name = true;
+#else
+  const bool drive_letter_is_root_name = false;
+#endif
+  VERIFY( path("c:/").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo/").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo/bar").is_absolute() == drive_letter_is_root_name );
+  VERIFY( path("c:/foo/bar/").is_absolute() == drive_letter_is_root_name );
+}
+
+int
+main()
+{
+  test01();
+}