libstdc++: Fix path::generic_string allocator handling (PR 94242)

It's not possible to construct a path::string_type from an allocator of
a different type. Create the correct specialization of basic_string, and
adjust path::_S_str_convert to use a basic_string_view so that it is
independent of the allocator type.

	PR libstdc++/94242
	* include/bits/fs_path.h (path::_S_str_convert): Replace first
	parameter with basic_string_view so that strings with different
	allocators can be accepted.
	(path::generic_string<C,T,A>()): Use basic_string object that uses the
	right allocator type.
	* testsuite/27_io/filesystem/path/generic/94242.cc: New test.
	* testsuite/27_io/filesystem/path/generic/generic_string.cc: Improve
	test coverage.

libstdc++-v3/ChangeLog

@@ -1,3 +1,15 @@
+2020-03-21  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/94242
+	* include/bits/fs_path.h (path::_S_str_convert): Replace first
+	parameter with basic_string_view so that strings with different
+	allocators can be accepted.
+	(path::generic_string<C,T,A>()): Use basic_string object that uses the
+	right allocator type.
+	* testsuite/27_io/filesystem/path/generic/94242.cc: New test.
+	* testsuite/27_io/filesystem/path/generic/generic_string.cc: Improve
+	test coverage.
+
 2020-03-18  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/94033

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

@@ -579,7 +579,7 @@ namespace __detail
 
     template<typename _CharT, typename _Traits, typename _Allocator>
       static basic_string<_CharT, _Traits, _Allocator>
-      _S_str_convert(const string_type&, const _Allocator& __a);
+      _S_str_convert(basic_string_view<value_type>, const _Allocator&);
 
     void _M_split_cmpts();
 
@@ -1015,7 +1015,8 @@ namespace __detail
   /// @cond undocumented
   template<typename _CharT, typename _Traits, typename _Allocator>
     std::basic_string<_CharT, _Traits, _Allocator>
-    path::_S_str_convert(const string_type& __str, const _Allocator& __a)
+    path::_S_str_convert(basic_string_view<value_type> __str,
+			 const _Allocator& __a)
     {
       static_assert(!is_same_v<_CharT, value_type>);
 
@@ -1126,7 +1127,9 @@ namespace __detail
 #else
       const value_type __slash = '/';
 #endif
-      string_type __str(__a);
+      using _Alloc2 = typename allocator_traits<_Allocator>::template
+	rebind_alloc<value_type>;
+      basic_string<value_type, char_traits<value_type>, _Alloc2> __str(__a);
 
       if (_M_type() == _Type::_Root_dir)
 	__str.assign(1, __slash);
@@ -1145,7 +1148,7 @@ namespace __detail
 #endif
 	      if (__add_slash)
 		__str += __slash;
-	      __str += __elem._M_pathname;
+	      __str += basic_string_view<value_type>(__elem._M_pathname);
 	      __add_slash = __elem._M_type() == _Type::_Filename;
 	    }
 	}

libstdc++-v3/testsuite/27_io/filesystem/path/generic/94242.cc

@@ -0,0 +1,52 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++17 } }
+
+// Copyright (C) 2020 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/>.
+
+// C++17 30.10.7.4.7 path generic format observers [fs.path.generic.obs]
+
+#include <filesystem>
+#include <testsuite_allocator.h>
+
+using std::filesystem::path;
+using __gnu_test::SimpleAllocator;
+
+void
+test01()
+{
+  path p = "//foo//bar//.";
+  using C = path::value_type;
+  auto g = p.generic_string<C, std::char_traits<C>, SimpleAllocator<C>>();
+  VERIFY( g == path("/foo/bar/.").c_str() );
+}
+
+void
+test02()
+{
+  path p = "//foo//bar//.";
+  using C = char16_t;
+  auto g = p.generic_string<C, std::char_traits<C>, SimpleAllocator<C>>();
+  VERIFY( g == u"/foo/bar/." );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}

libstdc++-v3/testsuite/27_io/filesystem/path/generic/generic_string.cc

@@ -21,6 +21,7 @@
 // C++17 30.10.7.4.7 path generic format observers [fs.path.generic.obs]
 
 #include <filesystem>
+#include <testsuite_fs.h>
 #include <testsuite_hooks.h>
 
 using std::filesystem::path;
@@ -34,11 +35,15 @@ test01()
 #ifdef __CYGWIN__
   VERIFY( path("//a").generic_string() == "//a" );
   VERIFY( path("//a/").generic_string() == "//a/" );
+  VERIFY( path("//a//").generic_string() == "//a/" );
   VERIFY( path("//a/b").generic_string() == "//a/b" );
+  VERIFY( path("//a//b").generic_string() == "//a/b" );
 #else
   VERIFY( path("//a").generic_string() == "/a" );
   VERIFY( path("//a/").generic_string() == "/a/" );
+  VERIFY( path("//a//").generic_string() == "/a/" );
   VERIFY( path("//a/b").generic_string() == "/a/b" );
+  VERIFY( path("//a//b").generic_string() == "/a/b" );
 #endif
   VERIFY( path("/a//b").generic_string() == "/a/b" );
   VERIFY( path("/a//b/").generic_string() == "/a/b/" );
@@ -57,9 +62,36 @@ test02()
   }
 }
 
+void
+test03()
+{
+  for (path p : __gnu_test::test_paths)
+  {
+    // A path constructed from the generic format string should compare equal
+    // to the original, because they represent the same path.
+    VERIFY( path(p.generic_string()) == p );
+    VERIFY( path(p.generic_wstring()) == p );
+    VERIFY( path(p.generic_u8string()) == p );
+    VERIFY( path(p.generic_u16string()) == p );
+    VERIFY( path(p.generic_u32string()) == p );
+  }
+
+  for (path p : { "a///b//c", "///a//b//c", "a:b//c", "a://b///c" })
+  {
+    // A path constructed from the generic format string should compare equal
+    // to the original, because they represent the same path.
+    VERIFY( path(p.generic_string()) == p );
+    VERIFY( path(p.generic_wstring()) == p );
+    VERIFY( path(p.generic_u8string()) == p );
+    VERIFY( path(p.generic_u16string()) == p );
+    VERIFY( path(p.generic_u32string()) == p );
+  }
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }