libstdc++: Implement C++20 features for <sstream>

New ctors and ::view() accessor for -
  * basic_stingbuf
  * basic_istringstream
  * basic_ostringstream
  * basic_stringstreamm

New ::get_allocator() accessor for basic_stringbuf.

libstdc++-v3/ChangeLog:
	* acinclude.m4 (glibcxx_SUBDIRS): Add src/c++20.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.29): New symbols.
	* configure: Regenerate.
	* include/std/sstream:
	(basic_stringbuf::basic_stringbuf(allocator const&)): New constructor.
	(basic_stringbuf::basic_stringbuf(openmode, allocator const&)): Likewise.
	(basic_stringbuf::basic_stringbuf(basic_string&&, openmode)): Likewise.
	(basic_stringbuf::basic_stringbuf(basic_stringbuf&&, allocator const&)):
	Likewise.
	(basic_stringbuf::get_allocator()): New method.
	(basic_stringbuf::view()): Likewise.
	(basic_istringstream::basic_istringstream(basic_string&&, openmode)):
	New constructor.
	(basic_istringstream::basic_istringstream(openmode, allocator const&)):
	Likewise
	(basic_istringstream::view()): New method.
	(basic_ostringstream::basic_ostringstream(basic_string&&, openmode)):
	New constructor.
	(basic_ostringstream::basic_ostringstream(openmode, allocator const&)):
	Likewise
	(basic_ostringstream::view()): New method.
	(basic_stringstream::basic_stringstream(basic_string&&, openmode)):
	New constructor.
	(basic_stringstream::basic_stringstream(openmode, allocator const&)):
	Likewise
	(basic_stringstream::view()): New method.
	* src/Makefile.in: Add c++20 directory.
	* src/Makefile.am: Regenerate.
	* src/c++20/Makefile.am: Add makefile for new sub-directory.
	* src/c++20/Makefile.in: Generate.
	* src/c++20/sstream-inst.cc: New file defining explicit
	instantiations for basic_stringbuf, basic_istringstream,
	basic_ostringstream, and basic_stringstream member functions
	added in C++20.
	* testsuite/27_io/basic_stringbuf/cons/char/2.cc: New test.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/2.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/view/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_istringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_istringstream/cons/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_istringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_istringstream/view/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/cons/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/view/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/cons/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/view/wchar_t/1.cc: Likewise.

2 files changed, 226 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/27_io/basic_stringbuf/cons/char/2.cc b/libstdc++-v3/testsuite/27_io/basic_stringbuf/cons/char/2.cc
new file mode 100644
index 00000000000..ce669358c85
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_stringbuf/cons/char/2.cc
@@ -0,0 +1,121 @@
+// 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/>.
+
+// 27.7.1.1  basic_stringbuf constructors  [lib.stringbuf.cons]
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <sstream>
+#include <string>
+#include <testsuite_allocator.h>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  {
+    using alloc_type = __gnu_test::uneq_allocator<char>;
+    using sbuf_t = std::basic_stringbuf<char, std::char_traits<char>,
+						alloc_type>;
+
+    alloc_type aa;
+    sbuf_t sbuf1(aa);
+    VERIFY( aa == sbuf1.get_allocator() );
+
+    alloc_type aaa(42);
+    sbuf_t sbuf2(aaa);
+    VERIFY( aaa == sbuf2.get_allocator() );
+
+    VERIFY( sbuf1.get_allocator() != sbuf2.get_allocator() );
+  }
+
+  std::stringbuf::allocator_type a;
+  {
+    std::stringbuf sbuf(std::ios_base::in, a);
+  }
+
+  {
+    std::stringbuf sbuf(a);
+  }
+}
+
+auto const cstr = "This is a test";
+
+void
+test02()
+{
+  std::string s1(cstr);
+  std::stringbuf sbuf(std::move(s1));
+  VERIFY( s1.empty() );
+
+  std::string s2(cstr);
+  VERIFY( sbuf.str() == s2 );
+}
+
+void
+test03()
+{
+  using alloc_type = __gnu_test::tracker_allocator<char>;
+  using str_type = std::basic_string<char, std::char_traits<char>, alloc_type>;
+
+  auto const mode = std::ios_base::in | std::ios_base::out;
+  str_type s1(cstr);
+
+  {
+    std::stringbuf::allocator_type a;
+    std::stringbuf sbuf(s1, mode, a);
+    std::string s2(cstr);
+    VERIFY( sbuf.str() == s2 );
+  }
+
+  {
+    std::stringbuf sbuf(s1, mode);
+    std::string s2(cstr);
+    VERIFY( sbuf.str() == s2 );
+  }
+
+  {
+    std::stringbuf sbuf(s1);
+    std::string s2(cstr);
+    VERIFY( sbuf.str() == s2 );
+  }
+}
+
+void
+test04()
+{
+  std::stringbuf sbuf1(cstr);
+
+  std::stringbuf::allocator_type a;
+  std::stringbuf sbuf2(std::move(sbuf1), a);
+  VERIFY( sbuf1.str().empty() );
+
+  std::string s(cstr);
+  VERIFY( sbuf2.str() == s );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/27_io/basic_stringbuf/cons/wchar_t/2.cc b/libstdc++-v3/testsuite/27_io/basic_stringbuf/cons/wchar_t/2.cc
new file mode 100644
index 00000000000..e05acc42165
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_stringbuf/cons/wchar_t/2.cc
@@ -0,0 +1,105 @@
+// 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/>.
+
+// 27.7.1.1  basic_stringbuf constructors  [lib.stringbuf.cons]
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { dg-require-effective-target cxx11-abi }
+
+#include <sstream>
+#include <string>
+#include <testsuite_allocator.h>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::wstringbuf::allocator_type a;
+  {
+    std::wstringbuf sbuf(std::ios_base::in, a);
+  }
+
+  {
+    std::wstringbuf sbuf(a);
+  }
+}
+
+auto const cstr = L"This is a test";
+
+void
+test02()
+{
+  std::wstring s1(cstr);
+  std::wstringbuf sbuf(std::move(s1));
+  VERIFY( s1.empty() );
+
+  std::wstring s2(cstr);
+  VERIFY( sbuf.str() == s2 );
+}
+
+void
+test03()
+{
+  using alloc_type = __gnu_test::tracker_allocator<wchar_t>;
+  using str_type = std::basic_string<wchar_t, std::char_traits<wchar_t>, alloc_type>;
+
+  auto const mode = std::ios_base::in | std::ios_base::out;
+  str_type s1(cstr);
+
+  {
+    std::wstringbuf::allocator_type a;
+    std::wstringbuf sbuf(s1, mode, a);
+    std::wstring s2(cstr);
+    VERIFY( sbuf.str() == s2 );
+  }
+
+  {
+    std::wstringbuf sbuf(s1, mode);
+    std::wstring s2(cstr);
+    VERIFY( sbuf.str() == s2 );
+  }
+
+  {
+    std::wstringbuf sbuf(s1);
+    std::wstring s2(cstr);
+    VERIFY( sbuf.str() == s2 );
+  }
+}
+
+void
+test04()
+{
+  std::wstringbuf sbuf1(cstr);
+
+  std::wstringbuf::allocator_type a;
+  std::wstringbuf sbuf2(std::move(sbuf1), a);
+  VERIFY( sbuf1.str().empty() );
+
+  std::wstring s(cstr);
+  VERIFY( sbuf2.str() == s );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
+  return 0;
+}