libstdc++: Fix Unicode codecvt and add tests [PR86419]

Fixes the conversion from UTF-8 to UTF-16 to properly return partial
instead ok.
Fixes the conversion from UTF-16 to UTF-8 to properly return partial
instead ok.
Fixes the conversion from UTF-8 to UCS-2 to properly return partial
instead error.
Fixes the conversion from UTF-8 to UCS-2 to treat 4-byte UTF-8 sequences
as error just by seeing the leading byte.
Fixes UTF-8 decoding for all codecvts so they detect error at the end of
the input range when the last code point is also incomplete.

libstdc++-v3/ChangeLog:

	PR libstdc++/86419
	* src/c++11/codecvt.cc (read_utf8_code_point): Correctly detect
	errors in incomplete multibyte sequences.
	(utf16_in): Remove surrogates parameter. Fix conditions for
	returning partial.
	(utf16_out): Fix condition for returning partial.
	(ucs2_in): Do not pass surrogates argument to utf16_in.
	* testsuite/22_locale/codecvt/codecvt_unicode.cc: New test.
	* testsuite/22_locale/codecvt/codecvt_unicode.h: New header for
	tests.
	* testsuite/22_locale/codecvt/codecvt_unicode_wchar_t.cc: New
	test.

3 files changed, 1396 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode.cc b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode.cc
new file mode 100644
index 0000000..ae4b6c8
--- /dev/null
+++ b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode.cc
@@ -0,0 +1,68 @@
+// Copyright (C) 2020-2023 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-do run { target c++11 } }
+
+#include "codecvt_unicode.h"
+
+#include <codecvt>
+
+using namespace std;
+
+void
+test_utf8_utf32_codecvts ()
+{
+  using codecvt_c32 = codecvt<char32_t, char, mbstate_t>;
+  auto loc_c = locale::classic ();
+  VERIFY (has_facet<codecvt_c32> (loc_c));
+  auto &cvt = use_facet<codecvt_c32> (loc_c);
+  test_utf8_utf32_codecvts (cvt);
+
+  auto cvt_ptr = to_unique_ptr (new codecvt_utf8<char32_t> ());
+  test_utf8_utf32_codecvts (*cvt_ptr);
+}
+
+void
+test_utf8_utf16_codecvts ()
+{
+  using codecvt_c16 = codecvt<char16_t, char, mbstate_t>;
+  auto loc_c = locale::classic ();
+  VERIFY (has_facet<codecvt_c16> (loc_c));
+  auto &cvt = use_facet<codecvt_c16> (loc_c);
+  test_utf8_utf16_cvts (cvt);
+
+  auto cvt_ptr = to_unique_ptr (new codecvt_utf8_utf16<char16_t> ());
+  test_utf8_utf16_cvts (*cvt_ptr);
+
+  auto cvt_ptr2 = to_unique_ptr (new codecvt_utf8_utf16<char32_t> ());
+  test_utf8_utf16_cvts (*cvt_ptr2);
+}
+
+void
+test_utf8_ucs2_codecvts ()
+{
+  auto cvt_ptr = to_unique_ptr (new codecvt_utf8<char16_t> ());
+  test_utf8_ucs2_cvts (*cvt_ptr);
+}
+
+int
+main ()
+{
+  test_utf8_utf32_codecvts ();
+  test_utf8_utf16_codecvts ();
+  test_utf8_ucs2_codecvts ();
+}
diff --git a/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode.h b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode.h
new file mode 100644
index 0000000..99d1a468
--- /dev/null
+++ b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode.h
@@ -0,0 +1,1269 @@
+// Copyright (C) 2020-2023 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/>.
+
+#include <locale>
+#include <string>
+#include <memory>
+#include <testsuite_hooks.h>
+
+template <typename T>
+std::unique_ptr<T>
+to_unique_ptr (T *ptr)
+{
+  return std::unique_ptr<T> (ptr);
+}
+
+struct test_offsets_ok
+{
+  size_t in_size, out_size;
+};
+struct test_offsets_partial
+{
+  size_t in_size, out_size, expected_in_next, expected_out_next;
+};
+
+template <class CharT> struct test_offsets_error
+{
+  size_t in_size, out_size, expected_in_next, expected_out_next;
+  CharT replace_char;
+  size_t replace_pos;
+};
+
+template <class T, size_t N>
+auto constexpr array_size (const T (&)[N]) -> size_t
+{
+  return N;
+}
+
+template <class CharT>
+void
+utf8_to_utf32_in_ok (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char in[] = "bш\uAAAA\U0010AAAA";
+  const char32_t exp_literal[] = U"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  std::copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (in) == 11, "");
+  static_assert (array_size (exp_literal) == 5, "");
+  static_assert (array_size (exp) == 5, "");
+  VERIFY (char_traits<char>::length (in) == 10);
+  VERIFY (char_traits<char32_t>::length (exp_literal) == 4);
+  VERIFY (char_traits<CharT>::length (exp) == 4);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {3, 2}, {6, 3}, {10, 4}};
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp)] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res
+	= cvt.in (state, in, in + t.in_size, in_next, out, end (out), out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_utf32_in_partial (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char in[] = "bш\uAAAA\U0010AAAA";
+  const char32_t exp_literal[] = U"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  std::copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (in) == 11, "");
+  static_assert (array_size (exp_literal) == 5, "");
+  static_assert (array_size (exp) == 5, "");
+  VERIFY (char_traits<char>::length (in) == 10);
+  VERIFY (char_traits<char32_t>::length (exp_literal) == 4);
+  VERIFY (char_traits<CharT>::length (exp) == 4);
+
+  test_offsets_partial offsets[] = {
+    {1, 0, 0, 0}, // no space for first CP
+
+    {3, 1, 1, 1}, // no space for second CP
+    {2, 2, 1, 1}, // incomplete second CP
+    {2, 1, 1, 1}, // incomplete second CP, and no space for it
+
+    {6, 2, 3, 2}, // no space for third CP
+    {4, 3, 3, 2}, // incomplete third CP
+    {5, 3, 3, 2}, // incomplete third CP
+    {4, 2, 3, 2}, // incomplete third CP, and no space for it
+    {5, 2, 3, 2}, // incomplete third CP, and no space for it
+
+    {10, 3, 6, 3}, // no space for fourth CP
+    {7, 4, 6, 3},  // incomplete fourth CP
+    {8, 4, 6, 3},  // incomplete fourth CP
+    {9, 4, 6, 3},  // incomplete fourth CP
+    {7, 3, 6, 3},  // incomplete fourth CP, and no space for it
+    {8, 3, 6, 3},  // incomplete fourth CP, and no space for it
+    {9, 3, 6, 3},  // incomplete fourth CP, and no space for it
+  };
+
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.partial);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_utf32_in_error (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char valid_in[] = "bш\uAAAA\U0010AAAA";
+  const char32_t exp_literal[] = U"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  std::copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (valid_in) == 11, "");
+  static_assert (array_size (exp_literal) == 5, "");
+  static_assert (array_size (exp) == 5, "");
+  VERIFY (char_traits<char>::length (valid_in) == 10);
+  VERIFY (char_traits<char32_t>::length (exp_literal) == 4);
+  VERIFY (char_traits<CharT>::length (exp) == 4);
+
+  test_offsets_error<char> offsets[] = {
+
+    // replace leading byte with invalid byte
+    {1, 4, 0, 0, '\xFF', 0},
+    {3, 4, 1, 1, '\xFF', 1},
+    {6, 4, 3, 2, '\xFF', 3},
+    {10, 4, 6, 3, '\xFF', 6},
+
+    // replace first trailing byte with ASCII byte
+    {3, 4, 1, 1, 'z', 2},
+    {6, 4, 3, 2, 'z', 4},
+    {10, 4, 6, 3, 'z', 7},
+
+    // replace first trailing byte with invalid byte
+    {3, 4, 1, 1, '\xFF', 2},
+    {6, 4, 3, 2, '\xFF', 4},
+    {10, 4, 6, 3, '\xFF', 7},
+
+    // replace second trailing byte with ASCII byte
+    {6, 4, 3, 2, 'z', 5},
+    {10, 4, 6, 3, 'z', 8},
+
+    // replace second trailing byte with invalid byte
+    {6, 4, 3, 2, '\xFF', 5},
+    {10, 4, 6, 3, '\xFF', 8},
+
+    // replace third trailing byte
+    {10, 4, 6, 3, 'z', 9},
+    {10, 4, 6, 3, '\xFF', 9},
+
+    // replace first trailing byte with ASCII byte, also incomplete at end
+    {5, 4, 3, 2, 'z', 4},
+    {8, 4, 6, 3, 'z', 7},
+    {9, 4, 6, 3, 'z', 7},
+
+    // replace first trailing byte with invalid byte, also incomplete at end
+    {5, 4, 3, 2, '\xFF', 4},
+    {8, 4, 6, 3, '\xFF', 7},
+    {9, 4, 6, 3, '\xFF', 7},
+
+    // replace second trailing byte with ASCII byte, also incomplete at end
+    {9, 4, 6, 3, 'z', 8},
+
+    // replace second trailing byte with invalid byte, also incomplete at end
+    {9, 4, 6, 3, '\xFF', 8},
+  };
+  for (auto t : offsets)
+    {
+      char in[array_size (valid_in)] = {};
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      char_traits<char>::copy (in, valid_in, array_size (valid_in));
+      in[t.replace_pos] = t.replace_char;
+
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.error);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_utf32_in (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf8_to_utf32_in_ok (cvt);
+  utf8_to_utf32_in_partial (cvt);
+  utf8_to_utf32_in_error (cvt);
+}
+
+template <class CharT>
+void
+utf32_to_utf8_out_ok (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char32_t in_literal[] = U"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+  CharT in[array_size (in_literal)] = {};
+  copy (begin (in_literal), end (in_literal), begin (in));
+
+  static_assert (array_size (in_literal) == 5, "");
+  static_assert (array_size (in) == 5, "");
+  static_assert (array_size (exp) == 11, "");
+  VERIFY (char_traits<char32_t>::length (in_literal) == 4);
+  VERIFY (char_traits<CharT>::length (in) == 4);
+  VERIFY (char_traits<char>::length (exp) == 10);
+
+  const test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {2, 3}, {3, 6}, {4, 10}};
+  for (auto t : offsets)
+    {
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<char>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf32_to_utf8_out_partial (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char32_t in_literal[] = U"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+  CharT in[array_size (in_literal)] = {};
+  copy (begin (in_literal), end (in_literal), begin (in));
+
+  static_assert (array_size (in_literal) == 5, "");
+  static_assert (array_size (in) == 5, "");
+  static_assert (array_size (exp) == 11, "");
+  VERIFY (char_traits<char32_t>::length (in_literal) == 4);
+  VERIFY (char_traits<CharT>::length (in) == 4);
+  VERIFY (char_traits<char>::length (exp) == 10);
+
+  const test_offsets_partial offsets[] = {
+    {1, 0, 0, 0}, // no space for first CP
+
+    {2, 1, 1, 1}, // no space for second CP
+    {2, 2, 1, 1}, // no space for second CP
+
+    {3, 3, 2, 3}, // no space for third CP
+    {3, 4, 2, 3}, // no space for third CP
+    {3, 5, 2, 3}, // no space for third CP
+
+    {4, 6, 3, 6}, // no space for fourth CP
+    {4, 7, 3, 6}, // no space for fourth CP
+    {4, 8, 3, 6}, // no space for fourth CP
+    {4, 9, 3, 6}, // no space for fourth CP
+  };
+  for (auto t : offsets)
+    {
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.partial);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<char>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf32_to_utf8_out_error (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  const char32_t valid_in[] = U"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+
+  static_assert (array_size (valid_in) == 5, "");
+  static_assert (array_size (exp) == 11, "");
+  VERIFY (char_traits<char32_t>::length (valid_in) == 4);
+  VERIFY (char_traits<char>::length (exp) == 10);
+
+  test_offsets_error<CharT> offsets[] = {{4, 10, 0, 0, 0x00110000, 0},
+					 {4, 10, 1, 1, 0x00110000, 1},
+					 {4, 10, 2, 3, 0x00110000, 2},
+					 {4, 10, 3, 6, 0x00110000, 3}};
+
+  for (auto t : offsets)
+    {
+      CharT in[array_size (valid_in)] = {};
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      copy (begin (valid_in), end (valid_in), begin (in));
+      in[t.replace_pos] = t.replace_char;
+
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.error);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<char>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf32_to_utf8_out (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf32_to_utf8_out_ok (cvt);
+  utf32_to_utf8_out_partial (cvt);
+  utf32_to_utf8_out_error (cvt);
+}
+
+template <class CharT>
+void
+test_utf8_utf32_codecvts (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf8_to_utf32_in (cvt);
+  utf32_to_utf8_out (cvt);
+}
+
+template <class CharT>
+void
+utf8_to_utf16_in_ok (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char in[] = "bш\uAAAA\U0010AAAA";
+  const char16_t exp_literal[] = u"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (in) == 11, "");
+  static_assert (array_size (exp_literal) == 6, "");
+  static_assert (array_size (exp) == 6, "");
+  VERIFY (char_traits<char>::length (in) == 10);
+  VERIFY (char_traits<char16_t>::length (exp_literal) == 5);
+  VERIFY (char_traits<CharT>::length (exp) == 5);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {3, 2}, {6, 3}, {10, 5}};
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp)] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res
+	= cvt.in (state, in, in + t.in_size, in_next, out, end (out), out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_utf16_in_partial (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char in[] = "bш\uAAAA\U0010AAAA";
+  const char16_t exp_literal[] = u"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (in) == 11, "");
+  static_assert (array_size (exp_literal) == 6, "");
+  static_assert (array_size (exp) == 6, "");
+  VERIFY (char_traits<char>::length (in) == 10);
+  VERIFY (char_traits<char16_t>::length (exp_literal) == 5);
+  VERIFY (char_traits<CharT>::length (exp) == 5);
+
+  test_offsets_partial offsets[] = {
+    {1, 0, 0, 0}, // no space for first CP
+
+    {3, 1, 1, 1}, // no space for second CP
+    {2, 2, 1, 1}, // incomplete second CP
+    {2, 1, 1, 1}, // incomplete second CP, and no space for it
+
+    {6, 2, 3, 2}, // no space for third CP
+    {4, 3, 3, 2}, // incomplete third CP
+    {5, 3, 3, 2}, // incomplete third CP
+    {4, 2, 3, 2}, // incomplete third CP, and no space for it
+    {5, 2, 3, 2}, // incomplete third CP, and no space for it
+
+    {10, 3, 6, 3}, // no space for fourth CP
+    {10, 4, 6, 3}, // no space for fourth CP
+    {7, 5, 6, 3},  // incomplete fourth CP
+    {8, 5, 6, 3},  // incomplete fourth CP
+    {9, 5, 6, 3},  // incomplete fourth CP
+    {7, 3, 6, 3},  // incomplete fourth CP, and no space for it
+    {8, 3, 6, 3},  // incomplete fourth CP, and no space for it
+    {9, 3, 6, 3},  // incomplete fourth CP, and no space for it
+    {7, 4, 6, 3},  // incomplete fourth CP, and no space for it
+    {8, 4, 6, 3},  // incomplete fourth CP, and no space for it
+    {9, 4, 6, 3},  // incomplete fourth CP, and no space for it
+
+  };
+
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.partial);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_utf16_in_error (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  const char valid_in[] = "bш\uAAAA\U0010AAAA";
+  const char16_t exp_literal[] = u"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (valid_in) == 11, "");
+  static_assert (array_size (exp_literal) == 6, "");
+  static_assert (array_size (exp) == 6, "");
+  VERIFY (char_traits<char>::length (valid_in) == 10);
+  VERIFY (char_traits<char16_t>::length (exp_literal) == 5);
+  VERIFY (char_traits<CharT>::length (exp) == 5);
+
+  test_offsets_error<char> offsets[] = {
+
+    // replace leading byte with invalid byte
+    {1, 5, 0, 0, '\xFF', 0},
+    {3, 5, 1, 1, '\xFF', 1},
+    {6, 5, 3, 2, '\xFF', 3},
+    {10, 5, 6, 3, '\xFF', 6},
+
+    // replace first trailing byte with ASCII byte
+    {3, 5, 1, 1, 'z', 2},
+    {6, 5, 3, 2, 'z', 4},
+    {10, 5, 6, 3, 'z', 7},
+
+    // replace first trailing byte with invalid byte
+    {3, 5, 1, 1, '\xFF', 2},
+    {6, 5, 3, 2, '\xFF', 4},
+    {10, 5, 6, 3, '\xFF', 7},
+
+    // replace second trailing byte with ASCII byte
+    {6, 5, 3, 2, 'z', 5},
+    {10, 5, 6, 3, 'z', 8},
+
+    // replace second trailing byte with invalid byte
+    {6, 5, 3, 2, '\xFF', 5},
+    {10, 5, 6, 3, '\xFF', 8},
+
+    // replace third trailing byte
+    {10, 5, 6, 3, 'z', 9},
+    {10, 5, 6, 3, '\xFF', 9},
+
+    // replace first trailing byte with ASCII byte, also incomplete at end
+    {5, 5, 3, 2, 'z', 4},
+    {8, 5, 6, 3, 'z', 7},
+    {9, 5, 6, 3, 'z', 7},
+
+    // replace first trailing byte with invalid byte, also incomplete at end
+    {5, 5, 3, 2, '\xFF', 4},
+    {8, 5, 6, 3, '\xFF', 7},
+    {9, 5, 6, 3, '\xFF', 7},
+
+    // replace second trailing byte with ASCII byte, also incomplete at end
+    {9, 5, 6, 3, 'z', 8},
+
+    // replace second trailing byte with invalid byte, also incomplete at end
+    {9, 5, 6, 3, '\xFF', 8},
+  };
+  for (auto t : offsets)
+    {
+      char in[array_size (valid_in)] = {};
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      char_traits<char>::copy (in, valid_in, array_size (valid_in));
+      in[t.replace_pos] = t.replace_char;
+
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.error);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_utf16_in (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf8_to_utf16_in_ok (cvt);
+  utf8_to_utf16_in_partial (cvt);
+  utf8_to_utf16_in_error (cvt);
+}
+
+template <class CharT>
+void
+utf16_to_utf8_out_ok (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char16_t in_literal[] = u"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+  CharT in[array_size (in_literal)];
+  copy (begin (in_literal), end (in_literal), begin (in));
+
+  static_assert (array_size (in_literal) == 6, "");
+  static_assert (array_size (exp) == 11, "");
+  static_assert (array_size (in) == 6, "");
+  VERIFY (char_traits<char16_t>::length (in_literal) == 5);
+  VERIFY (char_traits<char>::length (exp) == 10);
+  VERIFY (char_traits<CharT>::length (in) == 5);
+
+  const test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {2, 3}, {3, 6}, {5, 10}};
+  for (auto t : offsets)
+    {
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<char>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf16_to_utf8_out_partial (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char16_t in_literal[] = u"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+  CharT in[array_size (in_literal)];
+  copy (begin (in_literal), end (in_literal), begin (in));
+
+  static_assert (array_size (in_literal) == 6, "");
+  static_assert (array_size (exp) == 11, "");
+  static_assert (array_size (in) == 6, "");
+  VERIFY (char_traits<char16_t>::length (in_literal) == 5);
+  VERIFY (char_traits<char>::length (exp) == 10);
+  VERIFY (char_traits<CharT>::length (in) == 5);
+
+  const test_offsets_partial offsets[] = {
+    {1, 0, 0, 0}, // no space for first CP
+
+    {2, 1, 1, 1}, // no space for second CP
+    {2, 2, 1, 1}, // no space for second CP
+
+    {3, 3, 2, 3}, // no space for third CP
+    {3, 4, 2, 3}, // no space for third CP
+    {3, 5, 2, 3}, // no space for third CP
+
+    {5, 6, 3, 6}, // no space for fourth CP
+    {5, 7, 3, 6}, // no space for fourth CP
+    {5, 8, 3, 6}, // no space for fourth CP
+    {5, 9, 3, 6}, // no space for fourth CP
+
+    {4, 10, 3, 6}, // incomplete fourth CP
+
+    {4, 6, 3, 6}, // incomplete fourth CP, and no space for it
+    {4, 7, 3, 6}, // incomplete fourth CP, and no space for it
+    {4, 8, 3, 6}, // incomplete fourth CP, and no space for it
+    {4, 9, 3, 6}, // incomplete fourth CP, and no space for it
+  };
+  for (auto t : offsets)
+    {
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.partial);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<char>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf16_to_utf8_out_error (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  const char16_t valid_in[] = u"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+
+  static_assert (array_size (valid_in) == 6, "");
+  static_assert (array_size (exp) == 11, "");
+  VERIFY (char_traits<char16_t>::length (valid_in) == 5);
+  VERIFY (char_traits<char>::length (exp) == 10);
+
+  test_offsets_error<CharT> offsets[] = {
+    {5, 10, 0, 0, 0xD800, 0},
+    {5, 10, 0, 0, 0xDBFF, 0},
+    {5, 10, 0, 0, 0xDC00, 0},
+    {5, 10, 0, 0, 0xDFFF, 0},
+
+    {5, 10, 1, 1, 0xD800, 1},
+    {5, 10, 1, 1, 0xDBFF, 1},
+    {5, 10, 1, 1, 0xDC00, 1},
+    {5, 10, 1, 1, 0xDFFF, 1},
+
+    {5, 10, 2, 3, 0xD800, 2},
+    {5, 10, 2, 3, 0xDBFF, 2},
+    {5, 10, 2, 3, 0xDC00, 2},
+    {5, 10, 2, 3, 0xDFFF, 2},
+
+    // make the leading surrogate a trailing one
+    {5, 10, 3, 6, 0xDC00, 3},
+    {5, 10, 3, 6, 0xDFFF, 3},
+
+    // make the trailing surrogate a leading one
+    {5, 10, 3, 6, 0xD800, 4},
+    {5, 10, 3, 6, 0xDBFF, 4},
+
+    // make the trailing surrogate a BMP char
+    {5, 10, 3, 6, u'z', 4},
+  };
+
+  for (auto t : offsets)
+    {
+      CharT in[array_size (valid_in)] = {};
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      copy (begin (valid_in), end (valid_in), begin (in));
+      in[t.replace_pos] = t.replace_char;
+
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.error);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<char>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf16_to_utf8_out (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf16_to_utf8_out_ok (cvt);
+  utf16_to_utf8_out_partial (cvt);
+  utf16_to_utf8_out_error (cvt);
+}
+
+template <class CharT>
+void
+test_utf8_utf16_cvts (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf8_to_utf16_in (cvt);
+  utf16_to_utf8_out (cvt);
+}
+
+template <class CharT>
+void
+utf8_to_ucs2_in_ok (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const char in[] = "bш\uAAAA";
+  const char16_t exp_literal[] = u"bш\uAAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (in) == 7, "");
+  static_assert (array_size (exp_literal) == 4, "");
+  static_assert (array_size (exp) == 4, "");
+  VERIFY (char_traits<char>::length (in) == 6);
+  VERIFY (char_traits<char16_t>::length (exp_literal) == 3);
+  VERIFY (char_traits<CharT>::length (exp) == 3);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {3, 2}, {6, 3}};
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp)] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res
+	= cvt.in (state, in, in + t.in_size, in_next, out, end (out), out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_ucs2_in_partial (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const char in[] = "bш\uAAAA";
+  const char16_t exp_literal[] = u"bш\uAAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (in) == 7, "");
+  static_assert (array_size (exp_literal) == 4, "");
+  static_assert (array_size (exp) == 4, "");
+  VERIFY (char_traits<char>::length (in) == 6);
+  VERIFY (char_traits<char16_t>::length (exp_literal) == 3);
+  VERIFY (char_traits<CharT>::length (exp) == 3);
+
+  test_offsets_partial offsets[] = {
+    {1, 0, 0, 0}, // no space for first CP
+
+    {3, 1, 1, 1}, // no space for second CP
+    {2, 2, 1, 1}, // incomplete second CP
+    {2, 1, 1, 1}, // incomplete second CP, and no space for it
+
+    {6, 2, 3, 2}, // no space for third CP
+    {4, 3, 3, 2}, // incomplete third CP
+    {5, 3, 3, 2}, // incomplete third CP
+    {4, 2, 3, 2}, // incomplete third CP, and no space for it
+    {5, 2, 3, 2}, // incomplete third CP, and no space for it
+  };
+
+  for (auto t : offsets)
+    {
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.partial);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_ucs2_in_error (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  const char valid_in[] = "bш\uAAAA\U0010AAAA";
+  const char16_t exp_literal[] = u"bш\uAAAA\U0010AAAA";
+  CharT exp[array_size (exp_literal)] = {};
+  copy (begin (exp_literal), end (exp_literal), begin (exp));
+
+  static_assert (array_size (valid_in) == 11, "");
+  static_assert (array_size (exp_literal) == 6, "");
+  static_assert (array_size (exp) == 6, "");
+  VERIFY (char_traits<char>::length (valid_in) == 10);
+  VERIFY (char_traits<char16_t>::length (exp_literal) == 5);
+  VERIFY (char_traits<CharT>::length (exp) == 5);
+
+  test_offsets_error<char> offsets[] = {
+
+    // replace leading byte with invalid byte
+    {1, 5, 0, 0, '\xFF', 0},
+    {3, 5, 1, 1, '\xFF', 1},
+    {6, 5, 3, 2, '\xFF', 3},
+    {10, 5, 6, 3, '\xFF', 6},
+
+    // replace first trailing byte with ASCII byte
+    {3, 5, 1, 1, 'z', 2},
+    {6, 5, 3, 2, 'z', 4},
+    {10, 5, 6, 3, 'z', 7},
+
+    // replace first trailing byte with invalid byte
+    {3, 5, 1, 1, '\xFF', 2},
+    {6, 5, 3, 2, '\xFF', 4},
+    {10, 5, 6, 3, '\xFF', 7},
+
+    // replace second trailing byte with ASCII byte
+    {6, 5, 3, 2, 'z', 5},
+    {10, 5, 6, 3, 'z', 8},
+
+    // replace second trailing byte with invalid byte
+    {6, 5, 3, 2, '\xFF', 5},
+    {10, 5, 6, 3, '\xFF', 8},
+
+    // replace third trailing byte
+    {10, 5, 6, 3, 'z', 9},
+    {10, 5, 6, 3, '\xFF', 9},
+
+    // When we see a leading byte of 4-byte CP, we should return error, no
+    // matter if it is incomplete at the end or has errors in the trailing
+    // bytes.
+
+    // Don't replace anything, show full 4-byte CP
+    {10, 4, 6, 3, 'b', 0},
+    {10, 5, 6, 3, 'b', 0},
+
+    // Don't replace anything, show incomplete 4-byte CP at the end
+    {7, 4, 6, 3, 'b', 0}, // incomplete fourth CP
+    {8, 4, 6, 3, 'b', 0}, // incomplete fourth CP
+    {9, 4, 6, 3, 'b', 0}, // incomplete fourth CP
+    {7, 5, 6, 3, 'b', 0}, // incomplete fourth CP
+    {8, 5, 6, 3, 'b', 0}, // incomplete fourth CP
+    {9, 5, 6, 3, 'b', 0}, // incomplete fourth CP
+
+    // replace first trailing byte with ASCII byte, also incomplete at end
+    {5, 5, 3, 2, 'z', 4},
+
+    // replace first trailing byte with invalid byte, also incomplete at end
+    {5, 5, 3, 2, '\xFF', 4},
+
+    // replace first trailing byte with ASCII byte, also incomplete at end
+    {8, 5, 6, 3, 'z', 7},
+    {9, 5, 6, 3, 'z', 7},
+
+    // replace first trailing byte with invalid byte, also incomplete at end
+    {8, 5, 6, 3, '\xFF', 7},
+    {9, 5, 6, 3, '\xFF', 7},
+
+    // replace second trailing byte with ASCII byte, also incomplete at end
+    {9, 5, 6, 3, 'z', 8},
+
+    // replace second trailing byte with invalid byte, also incomplete at end
+    {9, 5, 6, 3, '\xFF', 8},
+  };
+  for (auto t : offsets)
+    {
+      char in[array_size (valid_in)] = {};
+      CharT out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      char_traits<char>::copy (in, valid_in, array_size (valid_in));
+      in[t.replace_pos] = t.replace_char;
+
+      auto state = mbstate_t{};
+      auto in_next = (const char *) nullptr;
+      auto out_next = (CharT *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.in (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		    out_next);
+      VERIFY (res == cvt.error);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<CharT>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+utf8_to_ucs2_in (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf8_to_ucs2_in_ok (cvt);
+  utf8_to_ucs2_in_partial (cvt);
+  utf8_to_ucs2_in_error (cvt);
+}
+
+template <class CharT>
+void
+ucs2_to_utf8_out_ok (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const char16_t in_literal[] = u"bш\uAAAA";
+  const char exp[] = "bш\uAAAA";
+  CharT in[array_size (in_literal)] = {};
+  copy (begin (in_literal), end (in_literal), begin (in));
+
+  static_assert (array_size (in_literal) == 4, "");
+  static_assert (array_size (exp) == 7, "");
+  static_assert (array_size (in) == 4, "");
+  VERIFY (char_traits<char16_t>::length (in_literal) == 3);
+  VERIFY (char_traits<char>::length (exp) == 6);
+  VERIFY (char_traits<CharT>::length (in) == 3);
+
+  const test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {2, 3}, {3, 6}};
+  for (auto t : offsets)
+    {
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.ok);
+      VERIFY (in_next == in + t.in_size);
+      VERIFY (out_next == out + t.out_size);
+      VERIFY (char_traits<char>::compare (out, exp, t.out_size) == 0);
+      if (t.out_size < array_size (out))
+	VERIFY (out[t.out_size] == 0);
+    }
+}
+
+template <class CharT>
+void
+ucs2_to_utf8_out_partial (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const char16_t in_literal[] = u"bш\uAAAA";
+  const char exp[] = "bш\uAAAA";
+  CharT in[array_size (in_literal)] = {};
+  copy (begin (in_literal), end (in_literal), begin (in));
+
+  static_assert (array_size (in_literal) == 4, "");
+  static_assert (array_size (exp) == 7, "");
+  static_assert (array_size (in) == 4, "");
+  VERIFY (char_traits<char16_t>::length (in_literal) == 3);
+  VERIFY (char_traits<char>::length (exp) == 6);
+  VERIFY (char_traits<CharT>::length (in) == 3);
+
+  const test_offsets_partial offsets[] = {
+    {1, 0, 0, 0}, // no space for first CP
+
+    {2, 1, 1, 1}, // no space for second CP
+    {2, 2, 1, 1}, // no space for second CP
+
+    {3, 3, 2, 3}, // no space for third CP
+    {3, 4, 2, 3}, // no space for third CP
+    {3, 5, 2, 3}, // no space for third CP
+  };
+  for (auto t : offsets)
+    {
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.partial);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<char>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+ucs2_to_utf8_out_error (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  using namespace std;
+  const char16_t valid_in[] = u"bш\uAAAA\U0010AAAA";
+  const char exp[] = "bш\uAAAA\U0010AAAA";
+
+  static_assert (array_size (valid_in) == 6, "");
+  static_assert (array_size (exp) == 11, "");
+  VERIFY (char_traits<char16_t>::length (valid_in) == 5);
+  VERIFY (char_traits<char>::length (exp) == 10);
+
+  test_offsets_error<CharT> offsets[] = {
+    {5, 10, 0, 0, 0xD800, 0},
+    {5, 10, 0, 0, 0xDBFF, 0},
+    {5, 10, 0, 0, 0xDC00, 0},
+    {5, 10, 0, 0, 0xDFFF, 0},
+
+    {5, 10, 1, 1, 0xD800, 1},
+    {5, 10, 1, 1, 0xDBFF, 1},
+    {5, 10, 1, 1, 0xDC00, 1},
+    {5, 10, 1, 1, 0xDFFF, 1},
+
+    {5, 10, 2, 3, 0xD800, 2},
+    {5, 10, 2, 3, 0xDBFF, 2},
+    {5, 10, 2, 3, 0xDC00, 2},
+    {5, 10, 2, 3, 0xDFFF, 2},
+
+    // dont replace anything, just show the surrogate pair
+    {5, 10, 3, 6, u'b', 0},
+
+    // make the leading surrogate a trailing one
+    {5, 10, 3, 6, 0xDC00, 3},
+    {5, 10, 3, 6, 0xDFFF, 3},
+
+    // make the trailing surrogate a leading one
+    {5, 10, 3, 6, 0xD800, 4},
+    {5, 10, 3, 6, 0xDBFF, 4},
+
+    // make the trailing surrogate a BMP char
+    {5, 10, 3, 6, u'z', 4},
+
+    {5, 7, 3, 6, u'b', 0}, // no space for fourth CP
+    {5, 8, 3, 6, u'b', 0}, // no space for fourth CP
+    {5, 9, 3, 6, u'b', 0}, // no space for fourth CP
+
+    {4, 10, 3, 6, u'b', 0}, // incomplete fourth CP
+    {4, 7, 3, 6, u'b', 0},  // incomplete fourth CP, and no space for it
+    {4, 8, 3, 6, u'b', 0},  // incomplete fourth CP, and no space for it
+    {4, 9, 3, 6, u'b', 0},  // incomplete fourth CP, and no space for it
+
+  };
+
+  for (auto t : offsets)
+    {
+      CharT in[array_size (valid_in)] = {};
+      char out[array_size (exp) - 1] = {};
+      VERIFY (t.in_size <= array_size (in));
+      VERIFY (t.out_size <= array_size (out));
+      VERIFY (t.expected_in_next <= t.in_size);
+      VERIFY (t.expected_out_next <= t.out_size);
+      copy (begin (valid_in), end (valid_in), begin (in));
+      in[t.replace_pos] = t.replace_char;
+
+      auto state = mbstate_t{};
+      auto in_next = (const CharT *) nullptr;
+      auto out_next = (char *) nullptr;
+      auto res = codecvt_base::result ();
+
+      res = cvt.out (state, in, in + t.in_size, in_next, out, out + t.out_size,
+		     out_next);
+      VERIFY (res == cvt.error);
+      VERIFY (in_next == in + t.expected_in_next);
+      VERIFY (out_next == out + t.expected_out_next);
+      VERIFY (char_traits<char>::compare (out, exp, t.expected_out_next) == 0);
+      if (t.expected_out_next < array_size (out))
+	VERIFY (out[t.expected_out_next] == 0);
+    }
+}
+
+template <class CharT>
+void
+ucs2_to_utf8_out (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  ucs2_to_utf8_out_ok (cvt);
+  ucs2_to_utf8_out_partial (cvt);
+  ucs2_to_utf8_out_error (cvt);
+}
+
+template <class CharT>
+void
+test_utf8_ucs2_cvts (const std::codecvt<CharT, char, mbstate_t> &cvt)
+{
+  utf8_to_ucs2_in (cvt);
+  ucs2_to_utf8_out (cvt);
+}
diff --git a/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode_wchar_t.cc b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode_wchar_t.cc
new file mode 100644
index 0000000..1695049
--- /dev/null
+++ b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_unicode_wchar_t.cc
@@ -0,0 +1,59 @@
+// Copyright (C) 2020-2023 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-do run { target c++11 } }
+
+#include "codecvt_unicode.h"
+
+#include <codecvt>
+
+using namespace std;
+
+void
+test_utf8_utf32_codecvts ()
+{
+#if __SIZEOF_WCHAR_T__ == 4
+  auto cvt_ptr = to_unique_ptr (new codecvt_utf8<wchar_t> ());
+  test_utf8_utf32_codecvts (*cvt_ptr);
+#endif
+}
+
+void
+test_utf8_utf16_codecvts ()
+{
+#if __SIZEOF_WCHAR_T__ >= 2
+  auto cvt_ptr = to_unique_ptr (new codecvt_utf8_utf16<wchar_t> ());
+  test_utf8_utf16_cvts (*cvt_ptr);
+#endif
+}
+
+void
+test_utf8_ucs2_codecvts ()
+{
+#if __SIZEOF_WCHAR_T__ == 2
+  auto cvt_ptr = to_unique_ptr (new codecvt_utf8<wchar_t> ());
+  test_utf8_ucs2_cvts (*cvt_ptr);
+#endif
+}
+
+int
+main ()
+{
+  test_utf8_utf32_codecvts ();
+  test_utf8_utf16_codecvts ();
+  test_utf8_ucs2_codecvts ();
+}