[libcxx-commits] [libcxx] 390840f - [libc++] Fix UTF-8 decoding in codecvts (#68442)

via libcxx-commits libcxx-commits at lists.llvm.org
Mon Nov 27 09:31:07 PST 2023


Author: Dimitrij Mijoski
Date: 2023-11-27T18:31:03+01:00
New Revision: 390840fbdf280b68a5498af952926ffcd678954e

URL: https://github.com/llvm/llvm-project/commit/390840fbdf280b68a5498af952926ffcd678954e
DIFF: https://github.com/llvm/llvm-project/commit/390840fbdf280b68a5498af952926ffcd678954e.diff

LOG: [libc++] Fix UTF-8 decoding in codecvts (#68442)

This patch fixes one case where the decoding member function `in()` was
returning `partial` instead of `error`. Additionally, it adds large
testsuite that tests all `codecvt` facets that were added in C++11 and
in C++20. The testsuite covers this bug.

Fixes #60177.

Added: 
    libcxx/test/std/localization/codecvt_unicode.pass.cpp

Modified: 
    libcxx/src/locale.cpp

Removed: 
    


################################################################################
diff  --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
index 2cb75f81b36da33..663f4124e39c662 100644
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -1972,10 +1972,9 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
         }
         else if (c1 < 0xF0)
         {
-            if (frm_end-frm_nxt < 3)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
             switch (c1)
             {
             case 0xE0:
@@ -1991,6 +1990,9 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
                     return codecvt_base::error;
                  break;
             }
+            if (frm_end-frm_nxt < 3)
+                return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
             if ((c3 & 0xC0) != 0x80)
                 return codecvt_base::error;
             uint16_t t = static_cast<uint16_t>(((c1 & 0x0F) << 12)
@@ -2003,11 +2005,9 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
         }
         else if (c1 < 0xF5)
         {
-            if (frm_end-frm_nxt < 4)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
-            uint8_t c4 = frm_nxt[3];
             switch (c1)
             {
             case 0xF0:
@@ -2023,8 +2023,16 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
                     return codecvt_base::error;
                  break;
             }
-            if ((c3 & 0xC0) != 0x80 || (c4 & 0xC0) != 0x80)
-                return codecvt_base::error;
+            if (frm_end-frm_nxt < 3)
+                 return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
+            if ((c3 & 0xC0) != 0x80)
+                 return codecvt_base::error;
+            if (frm_end-frm_nxt < 4)
+                 return codecvt_base::partial;
+            uint8_t c4 = frm_nxt[3];
+            if ((c4 & 0xC0) != 0x80)
+                 return codecvt_base::error;
             if (to_end-to_nxt < 2)
                 return codecvt_base::partial;
             if ((((c1 & 7UL) << 18) +
@@ -2093,10 +2101,9 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
         }
         else if (c1 < 0xF0)
         {
-            if (frm_end-frm_nxt < 3)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
             switch (c1)
             {
             case 0xE0:
@@ -2112,6 +2119,9 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
                     return codecvt_base::error;
                  break;
             }
+            if (frm_end-frm_nxt < 3)
+                 return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
             if ((c3 & 0xC0) != 0x80)
                 return codecvt_base::error;
             uint16_t t = static_cast<uint16_t>(((c1 & 0x0F) << 12)
@@ -2124,11 +2134,9 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
         }
         else if (c1 < 0xF5)
         {
-            if (frm_end-frm_nxt < 4)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
-            uint8_t c4 = frm_nxt[3];
             switch (c1)
             {
             case 0xF0:
@@ -2144,8 +2152,16 @@ utf8_to_utf16(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nx
                     return codecvt_base::error;
                  break;
             }
-            if ((c3 & 0xC0) != 0x80 || (c4 & 0xC0) != 0x80)
-                return codecvt_base::error;
+            if (frm_end-frm_nxt < 3)
+                 return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
+            if ((c3 & 0xC0) != 0x80)
+                 return codecvt_base::error;
+            if (frm_end-frm_nxt < 4)
+                 return codecvt_base::partial;
+            uint8_t c4 = frm_nxt[3];
+            if ((c4 & 0xC0) != 0x80)
+                 return codecvt_base::error;
             if (to_end-to_nxt < 2)
                 return codecvt_base::partial;
             if ((((c1 & 7UL) << 18) +
@@ -2371,10 +2387,9 @@ utf8_to_ucs4(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nxt
         }
         else if (c1 < 0xF0)
         {
-            if (frm_end-frm_nxt < 3)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
             switch (c1)
             {
             case 0xE0:
@@ -2390,6 +2405,9 @@ utf8_to_ucs4(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nxt
                     return codecvt_base::error;
                  break;
             }
+            if (frm_end-frm_nxt < 3)
+                 return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
             if ((c3 & 0xC0) != 0x80)
                 return codecvt_base::error;
             uint32_t t = static_cast<uint32_t>(((c1 & 0x0F) << 12)
@@ -2402,11 +2420,9 @@ utf8_to_ucs4(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nxt
         }
         else if (c1 < 0xF5)
         {
-            if (frm_end-frm_nxt < 4)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
-            uint8_t c4 = frm_nxt[3];
             switch (c1)
             {
             case 0xF0:
@@ -2422,8 +2438,16 @@ utf8_to_ucs4(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nxt
                     return codecvt_base::error;
                  break;
             }
-            if ((c3 & 0xC0) != 0x80 || (c4 & 0xC0) != 0x80)
-                return codecvt_base::error;
+            if (frm_end-frm_nxt < 3)
+                 return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
+            if ((c3 & 0xC0) != 0x80)
+                 return codecvt_base::error;
+            if (frm_end-frm_nxt < 4)
+                 return codecvt_base::partial;
+            uint8_t c4 = frm_nxt[3];
+            if ((c4 & 0xC0) != 0x80)
+                 return codecvt_base::error;
             uint32_t t = static_cast<uint32_t>(((c1 & 0x07) << 18)
                                              | ((c2 & 0x3F) << 12)
                                              | ((c3 & 0x3F) << 6)
@@ -2629,10 +2653,9 @@ utf8_to_ucs2(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nxt
         }
         else if (c1 < 0xF0)
         {
-            if (frm_end-frm_nxt < 3)
+            if (frm_end-frm_nxt < 2)
                 return codecvt_base::partial;
             uint8_t c2 = frm_nxt[1];
-            uint8_t c3 = frm_nxt[2];
             switch (c1)
             {
             case 0xE0:
@@ -2648,6 +2671,9 @@ utf8_to_ucs2(const uint8_t* frm, const uint8_t* frm_end, const uint8_t*& frm_nxt
                     return codecvt_base::error;
                  break;
             }
+            if (frm_end-frm_nxt < 3)
+                 return codecvt_base::partial;
+            uint8_t c3 = frm_nxt[2];
             if ((c3 & 0xC0) != 0x80)
                 return codecvt_base::error;
             uint16_t t = static_cast<uint16_t>(((c1 & 0x0F) << 12)

diff  --git a/libcxx/test/std/localization/codecvt_unicode.pass.cpp b/libcxx/test/std/localization/codecvt_unicode.pass.cpp
new file mode 100644
index 000000000000000..08ecc16ef4311eb
--- /dev/null
+++ b/libcxx/test/std/localization/codecvt_unicode.pass.cpp
@@ -0,0 +1,2229 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS -D_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT
+// XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0|13.0}}
+
+#include <algorithm>
+#include <cassert>
+#include <codecvt>
+#include <locale>
+
+#include "test_macros.h"
+
+struct test_offsets_ok {
+  size_t in_size;
+  size_t out_size;
+};
+struct test_offsets_partial {
+  size_t in_size;
+  size_t out_size;
+  size_t expected_in_next;
+  size_t expected_out_next;
+};
+
+template <class CharT>
+struct test_offsets_error {
+  size_t in_size;
+  size_t out_size;
+  size_t expected_in_next;
+  size_t expected_out_next;
+  CharT replace_char;
+  size_t replace_pos;
+};
+
+#define array_size(x) (sizeof(x) / sizeof(x)[0])
+
+using std::begin;
+using std::char_traits;
+using std::codecvt_base;
+using std::copy;
+using std::end;
+
+template <class InternT, class ExternT>
+void utf8_to_utf32_in_ok(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const unsigned char input[] = "b\u0448\uAAAA\U0010AAAA";
+  const char32_t expected[]   = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 5, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::length(exp) == 4);
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {3, 2}, {6, 3}, {10, 4}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t            = *it;
+    InternT out[array_size(exp)] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, end(out), out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, array_size(out));
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf32_in_partial(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const unsigned char input[] = "b\u0448\uAAAA\U0010AAAA";
+  const char32_t expected[]   = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 5, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf32_in_error(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP, 4-byte CP
+  const unsigned char input[] = "b\u0448\uD700\U0010AAAA";
+  const char32_t expected[]   = {'b', 0x0448, 0xD700, 0x10AAAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 5, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::length(exp) == 4);
+
+  // There are 5 classes of errors in UTF-8 decoding
+  // 1. Missing leading byte
+  // 2. Missing trailing byte
+  // 3. Surrogate CP
+  // 4. Overlong sequence
+  // 5. CP out of Unicode range
+  test_offsets_error<unsigned char> offsets[] = {
+
+      // 1. Missing leading byte. We will replace the leading byte with
+      // non-leading byte, such as a byte that is always invalid or a trailing
+      // byte.
+
+      // replace leading byte with invalid byte
+      {1, 4, 0, 0, 0xFF, 0},
+      {3, 4, 1, 1, 0xFF, 1},
+      {6, 4, 3, 2, 0xFF, 3},
+      {10, 4, 6, 3, 0xFF, 6},
+
+      // replace leading byte with trailing byte
+      {1, 4, 0, 0, 0b10101010, 0},
+      {3, 4, 1, 1, 0b10101010, 1},
+      {6, 4, 3, 2, 0b10101010, 3},
+      {10, 4, 6, 3, 0b10101010, 6},
+
+      // 2. Missing trailing byte. We will replace the trailing byte with
+      // non-trailing byte, such as a byte that is always invalid or a leading
+      // byte (simple ASCII byte in our case).
+
+      // 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, 0xFF, 2},
+      {6, 4, 3, 2, 0xFF, 4},
+      {10, 4, 6, 3, 0xFF, 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, 0xFF, 5},
+      {10, 4, 6, 3, 0xFF, 8},
+
+      // replace third trailing byte
+      {10, 4, 6, 3, 'z', 9},
+      {10, 4, 6, 3, 0xFF, 9},
+
+      // 2.1 The following test-cases raise doubt whether error or partial should
+      // be returned. For example, we have 4-byte sequence with valid leading
+      // byte. If we hide the last byte we need to return partial. But, if the
+      // second or third byte, which are visible to the call to codecvt, are
+      // malformed then error should be returned.
+
+      // 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, 0xFF, 4},
+      {8, 4, 6, 3, 0xFF, 7},
+      {9, 4, 6, 3, 0xFF, 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, 0xFF, 8},
+
+      // 3. Surrogate CP. We modify the second byte (first trailing) of the 3-byte
+      // CP U+D700
+      {6, 4, 3, 2, 0b10100000, 4}, // turn U+D700 into U+D800
+      {6, 4, 3, 2, 0b10101100, 4}, // turn U+D700 into U+DB00
+      {6, 4, 3, 2, 0b10110000, 4}, // turn U+D700 into U+DC00
+      {6, 4, 3, 2, 0b10111100, 4}, // turn U+D700 into U+DF00
+
+      // 4. Overlong sequence. The CPs in the input are chosen such as modifying
+      // just the leading byte is enough to make them overlong, i.e. for the
+      // 3-byte and 4-byte CP the second byte (first trailing) has enough leading
+      // zeroes.
+      {3, 4, 1, 1, 0b11000000, 1},  // make the 2-byte CP overlong
+      {3, 4, 1, 1, 0b11000001, 1},  // make the 2-byte CP overlong
+      {6, 4, 3, 2, 0b11100000, 3},  // make the 3-byte CP overlong
+      {10, 4, 6, 3, 0b11110000, 6}, // make the 4-byte CP overlong
+
+      // 5. CP above range
+      // turn U+10AAAA into U+14AAAA by changing its leading byte
+      {10, 4, 6, 3, 0b11110101, 6},
+      // turn U+10AAAA into U+11AAAA by changing its 2nd byte
+      {10, 4, 6, 3, 0b10011010, 7},
+  };
+  for (test_offsets_error<unsigned char>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<unsigned char> t = *it;
+    InternT out[array_size(exp) - 1]    = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    ExternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf32_in(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf8_to_utf32_in_ok(cvt);
+  utf8_to_utf32_in_partial(cvt);
+  utf8_to_utf32_in_error(cvt);
+}
+
+template <class InternT, class ExternT>
+void utf32_to_utf8_out_ok(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char32_t input[]         = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 5, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 4);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {2, 3}, {3, 6}, {4, 10}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<ExternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf32_to_utf8_out_partial(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char32_t input[]         = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 5, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 4);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<ExternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf32_to_utf8_out_error(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char32_t input[]         = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 5, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 4);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  test_offsets_error<InternT> offsets[] = {
+
+      // Surrogate CP
+      {4, 10, 0, 0, 0xD800, 0},
+      {4, 10, 1, 1, 0xDBFF, 1},
+      {4, 10, 2, 3, 0xDC00, 2},
+      {4, 10, 3, 6, 0xDFFF, 3},
+
+      // CP out of range
+      {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 (test_offsets_error<InternT>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<InternT> t    = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    InternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<ExternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT, class ExternT>
+void utf32_to_utf8_out(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf32_to_utf8_out_ok(cvt);
+  utf32_to_utf8_out_partial(cvt);
+  utf32_to_utf8_out_error(cvt);
+}
+
+template <class InternT, class ExternT>
+void test_utf8_utf32_cvt(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf8_to_utf32_in(cvt);
+  utf32_to_utf8_out(cvt);
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf16_in_ok(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const unsigned char input[] = "b\u0448\uAAAA\U0010AAAA";
+  const char16_t expected[]   = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 6, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::length(exp) == 5);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {3, 2}, {6, 3}, {10, 5}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t            = *it;
+    InternT out[array_size(exp)] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, end(out), out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, array_size(out));
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf16_in_partial(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const unsigned char input[] = "b\u0448\uAAAA\U0010AAAA";
+  const char16_t expected[]   = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 6, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf16_in_error(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP, 4-byte CP
+  const unsigned char input[] = "b\u0448\uD700\U0010AAAA";
+  const char16_t expected[]   = {'b', 0x0448, 0xD700, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 6, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::length(exp) == 5);
+
+  // There are 5 classes of errors in UTF-8 decoding
+  // 1. Missing leading byte
+  // 2. Missing trailing byte
+  // 3. Surrogate CP
+  // 4. Overlong sequence
+  // 5. CP out of Unicode range
+  test_offsets_error<unsigned char> offsets[] = {
+
+      // 1. Missing leading byte. We will replace the leading byte with
+      // non-leading byte, such as a byte that is always invalid or a trailing
+      // byte.
+
+      // replace leading byte with invalid byte
+      {1, 5, 0, 0, 0xFF, 0},
+      {3, 5, 1, 1, 0xFF, 1},
+      {6, 5, 3, 2, 0xFF, 3},
+      {10, 5, 6, 3, 0xFF, 6},
+
+      // replace leading byte with trailing byte
+      {1, 5, 0, 0, 0b10101010, 0},
+      {3, 5, 1, 1, 0b10101010, 1},
+      {6, 5, 3, 2, 0b10101010, 3},
+      {10, 5, 6, 3, 0b10101010, 6},
+
+      // 2. Missing trailing byte. We will replace the trailing byte with
+      // non-trailing byte, such as a byte that is always invalid or a leading
+      // byte (simple ASCII byte in our case).
+
+      // 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, 0xFF, 2},
+      {6, 5, 3, 2, 0xFF, 4},
+      {10, 5, 6, 3, 0xFF, 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, 0xFF, 5},
+      {10, 5, 6, 3, 0xFF, 8},
+
+      // replace third trailing byte
+      {10, 5, 6, 3, 'z', 9},
+      {10, 5, 6, 3, 0xFF, 9},
+
+      // 2.1 The following test-cases raise doubt whether error or partial should
+      // be returned. For example, we have 4-byte sequence with valid leading
+      // byte. If we hide the last byte we need to return partial. But, if the
+      // second or third byte, which are visible to the call to codecvt, are
+      // malformed then error should be returned.
+
+      // 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, 0xFF, 4},
+      {8, 5, 6, 3, 0xFF, 7},
+      {9, 5, 6, 3, 0xFF, 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, 0xFF, 8},
+
+      // 3. Surrogate CP. We modify the second byte (first trailing) of the 3-byte
+      // CP U+D700
+      {6, 5, 3, 2, 0b10100000, 4}, // turn U+D700 into U+D800
+      {6, 5, 3, 2, 0b10101100, 4}, // turn U+D700 into U+DB00
+      {6, 5, 3, 2, 0b10110000, 4}, // turn U+D700 into U+DC00
+      {6, 5, 3, 2, 0b10111100, 4}, // turn U+D700 into U+DF00
+
+      // 4. Overlong sequence. The CPs in the input are chosen such as modifying
+      // just the leading byte is enough to make them overlong, i.e. for the
+      // 3-byte and 4-byte CP the second byte (first trailing) has enough leading
+      // zeroes.
+      {3, 5, 1, 1, 0b11000000, 1},  // make the 2-byte CP overlong
+      {3, 5, 1, 1, 0b11000001, 1},  // make the 2-byte CP overlong
+      {6, 5, 3, 2, 0b11100000, 3},  // make the 3-byte CP overlong
+      {10, 5, 6, 3, 0b11110000, 6}, // make the 4-byte CP overlong
+
+      // 5. CP above range
+      // turn U+10AAAA into U+14AAAA by changing its leading byte
+      {10, 5, 6, 3, 0b11110101, 6},
+      // turn U+10AAAA into U+11AAAA by changing its 2nd byte
+      {10, 5, 6, 3, 0b10011010, 7},
+  };
+  for (test_offsets_error<unsigned char>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<unsigned char> t = *it;
+    InternT out[array_size(exp) - 1]    = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    ExternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_utf16_in(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf8_to_utf16_in_ok(cvt);
+  utf8_to_utf16_in_partial(cvt);
+  utf8_to_utf16_in_error(cvt);
+}
+
+template <class InternT, class ExternT>
+void utf16_to_utf8_out_ok(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char16_t input[]         = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 5);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {2, 3}, {3, 6}, {5, 10}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<ExternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf16_to_utf8_out_partial(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char16_t input[]         = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 5);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<ExternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf16_to_utf8_out_error(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP, 3-byte CP and 4-byte CP
+  const char16_t input[]         = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 5);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  // The only possible error in UTF-16 is unpaired surrogate code units.
+  // So we replace valid code points (scalar values) with lone surrogate CU.
+  test_offsets_error<InternT> 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, 'z', 4},
+  };
+
+  for (test_offsets_error<InternT>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<InternT> t    = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    InternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<ExternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT, class ExternT>
+void utf16_to_utf8_out(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf16_to_utf8_out_ok(cvt);
+  utf16_to_utf8_out_partial(cvt);
+  utf16_to_utf8_out_error(cvt);
+}
+
+template <class InternT, class ExternT>
+void test_utf8_utf16_cvt(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf8_to_utf16_in(cvt);
+  utf16_to_utf8_out(cvt);
+}
+
+template <class InternT, class ExternT>
+void utf8_to_ucs2_in_ok(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const unsigned char input[] = "b\u0448\uAAAA";
+  const char16_t expected[]   = {'b', 0x0448, 0xAAAA, 0};
+  static_assert(array_size(input) == 7, "");
+  static_assert(array_size(expected) == 4, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 6);
+  assert(char_traits<InternT>::length(exp) == 3);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {3, 2}, {6, 3}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t            = *it;
+    InternT out[array_size(exp)] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, end(out), out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, array_size(out));
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_ucs2_in_partial(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const unsigned char input[] = "b\u0448\uAAAA";
+  const char16_t expected[]   = {'b', 0x0448, 0xAAAA, 0};
+  static_assert(array_size(input) == 7, "");
+  static_assert(array_size(expected) == 4, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 6);
+  assert(char_traits<InternT>::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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_ucs2_in_error(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  const unsigned char input[] = "b\u0448\uD700\U0010AAAA";
+  const char16_t expected[]   = {'b', 0x0448, 0xD700, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 11, "");
+  static_assert(array_size(expected) == 6, "");
+
+  ExternT in[array_size(input)];
+  InternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<ExternT>::length(in) == 10);
+  assert(char_traits<InternT>::length(exp) == 5);
+
+  // There are 5 classes of errors in UTF-8 decoding
+  // 1. Missing leading byte
+  // 2. Missing trailing byte
+  // 3. Surrogate CP
+  // 4. Overlong sequence
+  // 5. CP out of Unicode range
+  test_offsets_error<unsigned char> offsets[] = {
+
+      // 1. Missing leading byte. We will replace the leading byte with
+      // non-leading byte, such as a byte that is always invalid or a trailing
+      // byte.
+
+      // replace leading byte with invalid byte
+      {1, 5, 0, 0, 0xFF, 0},
+      {3, 5, 1, 1, 0xFF, 1},
+      {6, 5, 3, 2, 0xFF, 3},
+      {10, 5, 6, 3, 0xFF, 6},
+
+      // replace leading byte with trailing byte
+      {1, 5, 0, 0, 0b10101010, 0},
+      {3, 5, 1, 1, 0b10101010, 1},
+      {6, 5, 3, 2, 0b10101010, 3},
+      {10, 5, 6, 3, 0b10101010, 6},
+
+      // 2. Missing trailing byte. We will replace the trailing byte with
+      // non-trailing byte, such as a byte that is always invalid or a leading
+      // byte (simple ASCII byte in our case).
+
+      // 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, 0xFF, 2},
+      {6, 5, 3, 2, 0xFF, 4},
+      {10, 5, 6, 3, 0xFF, 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, 0xFF, 5},
+      {10, 5, 6, 3, 0xFF, 8},
+
+      // replace third trailing byte
+      {10, 5, 6, 3, 'z', 9},
+      {10, 5, 6, 3, 0xFF, 9},
+
+      // 2.1 The following test-cases raise doubt whether error or partial should
+      // be returned. For example, we have 4-byte sequence with valid leading
+      // byte. If we hide the last byte we need to return partial. But, if the
+      // second or third byte, which are visible to the call to codecvt, are
+      // malformed then error should be returned.
+
+      // 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, 0xFF, 4},
+      {8, 5, 6, 3, 0xFF, 7},
+      {9, 5, 6, 3, 0xFF, 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, 0xFF, 8},
+
+      // 3. Surrogate CP. We modify the second byte (first trailing) of the 3-byte
+      // CP U+D700
+      {6, 5, 3, 2, 0b10100000, 4}, // turn U+D700 into U+D800
+      {6, 5, 3, 2, 0b10101100, 4}, // turn U+D700 into U+DB00
+      {6, 5, 3, 2, 0b10110000, 4}, // turn U+D700 into U+DC00
+      {6, 5, 3, 2, 0b10111100, 4}, // turn U+D700 into U+DF00
+
+      // 4. Overlong sequence. The CPs in the input are chosen such as modifying
+      // just the leading byte is enough to make them overlong, i.e. for the
+      // 3-byte and 4-byte CP the second byte (first trailing) has enough leading
+      // zeroes.
+      {3, 5, 1, 1, 0b11000000, 1},  // make the 2-byte CP overlong
+      {3, 5, 1, 1, 0b11000001, 1},  // make the 2-byte CP overlong
+      {6, 5, 3, 2, 0b11100000, 3},  // make the 3-byte CP overlong
+      {10, 5, 6, 3, 0b11110000, 6}, // make the 4-byte CP overlong
+
+      // 5. CP above range
+      // turn U+10AAAA into U+14AAAA by changing its leading byte
+      {10, 5, 6, 3, 0b11110101, 6},
+      // turn U+10AAAA into U+11AAAA by changing its 2nd byte
+      {10, 5, 6, 3, 0b10011010, 7},
+      // Don't replace anything, show full 4-byte CP U+10AAAA
+      {10, 4, 6, 3, 'b', 0},
+      {10, 5, 6, 3, 'b', 0},
+      // Don't replace anything, show incomplete 4-byte CP at the end. It's still
+      // out of UCS2 range just by seeing the first byte.
+      {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
+  };
+  for (test_offsets_error<unsigned char>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<unsigned char> t = *it;
+    InternT out[array_size(exp) - 1]    = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    ExternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const ExternT* in_next   = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT, class ExternT>
+void utf8_to_ucs2_in(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf8_to_ucs2_in_ok(cvt);
+  utf8_to_ucs2_in_partial(cvt);
+  utf8_to_ucs2_in_error(cvt);
+}
+
+template <class InternT, class ExternT>
+void ucs2_to_utf8_out_ok(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const char16_t input[]         = {'b', 0x0448, 0xAAAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA";
+  static_assert(array_size(input) == 4, "");
+  static_assert(array_size(expected) == 7, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 3);
+  assert(char_traits<ExternT>::length(exp) == 6);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 1}, {2, 3}, {3, 6}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<ExternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+  }
+}
+
+template <class InternT, class ExternT>
+void ucs2_to_utf8_out_partial(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  // UTF-8 string of 1-byte CP, 2-byte CP and 3-byte CP
+  const char16_t input[]         = {'b', 0x0448, 0xAAAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA";
+  static_assert(array_size(input) == 4, "");
+  static_assert(array_size(expected) == 7, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 3);
+  assert(char_traits<ExternT>::length(exp) == 6);
+
+  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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<ExternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+  }
+}
+
+template <class InternT, class ExternT>
+void ucs2_to_utf8_out_error(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  const char16_t input[]         = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const unsigned char expected[] = "b\u0448\uAAAA\U0010AAAA";
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 11, "");
+
+  InternT in[array_size(input)];
+  ExternT exp[array_size(expected)];
+  copy(begin(input), end(input), begin(in));
+  copy(begin(expected), end(expected), begin(exp));
+  assert(char_traits<InternT>::length(in) == 5);
+  assert(char_traits<ExternT>::length(exp) == 10);
+
+  test_offsets_error<InternT> offsets[] = {
+      {3, 6, 0, 0, 0xD800, 0},
+      {3, 6, 0, 0, 0xDBFF, 0},
+      {3, 6, 0, 0, 0xDC00, 0},
+      {3, 6, 0, 0, 0xDFFF, 0},
+
+      {3, 6, 1, 1, 0xD800, 1},
+      {3, 6, 1, 1, 0xDBFF, 1},
+      {3, 6, 1, 1, 0xDC00, 1},
+      {3, 6, 1, 1, 0xDFFF, 1},
+
+      {3, 6, 2, 3, 0xD800, 2},
+      {3, 6, 2, 3, 0xDBFF, 2},
+      {3, 6, 2, 3, 0xDC00, 2},
+      {3, 6, 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, 'z', 4},
+
+      // don't replace anything in the test cases bellow, just show the surrogate
+      // pair (fourth CP) fully or partially
+      {5, 10, 3, 6, 'b', 0},
+      {5, 7, 3, 6, 'b', 0}, // no space for fourth CP
+      {5, 8, 3, 6, 'b', 0}, // no space for fourth CP
+      {5, 9, 3, 6, 'b', 0}, // no space for fourth CP
+
+      {4, 10, 3, 6, 'b', 0}, // incomplete fourth CP
+      {4, 7, 3, 6, 'b', 0},  // incomplete fourth CP, and no space for it
+      {4, 8, 3, 6, 'b', 0},  // incomplete fourth CP, and no space for it
+      {4, 9, 3, 6, 'b', 0},  // incomplete fourth CP, and no space for it
+  };
+
+  for (test_offsets_error<InternT>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<InternT> t    = *it;
+    ExternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    InternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    ExternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<ExternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT, class ExternT>
+void ucs2_to_utf8_out(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  ucs2_to_utf8_out_ok(cvt);
+  ucs2_to_utf8_out_partial(cvt);
+  ucs2_to_utf8_out_error(cvt);
+}
+
+template <class InternT, class ExternT>
+void test_utf8_ucs2_cvt(const std::codecvt<InternT, ExternT, mbstate_t>& cvt) {
+  utf8_to_ucs2_in(cvt);
+  ucs2_to_utf8_out(cvt);
+}
+
+enum utf16_endianess { utf16_big_endian, utf16_little_endian };
+
+template <class Iter1, class Iter2>
+Iter2 utf16_to_bytes(Iter1 f, Iter1 l, Iter2 o, utf16_endianess e) {
+  if (e == utf16_big_endian)
+    for (; f != l; ++f) {
+      *o++ = (*f >> 8) & 0xFF;
+      *o++ = *f & 0xFF;
+    }
+  else
+    for (; f != l; ++f) {
+      *o++ = *f & 0xFF;
+      *o++ = (*f >> 8) & 0xFF;
+    }
+  return o;
+}
+
+template <class InternT>
+void utf16_to_utf32_in_ok(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const char32_t expected[] = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 5, "");
+
+  char in[array_size(input) * 2];
+  InternT exp[array_size(expected)];
+  utf16_to_bytes(begin(input), end(input), begin(in), endianess);
+  copy(begin(expected), end(expected), begin(exp));
+
+  test_offsets_ok offsets[] = {{0, 0}, {2, 1}, {4, 2}, {6, 3}, {10, 4}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t            = *it;
+    InternT out[array_size(exp)] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, end(out), out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, array_size(out));
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+}
+
+template <class InternT>
+void utf16_to_utf32_in_partial(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const char32_t expected[] = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 5, "");
+
+  char in[array_size(input) * 2];
+  InternT exp[array_size(expected)];
+  utf16_to_bytes(begin(input), end(input), begin(in), endianess);
+  copy(begin(expected), end(expected), begin(exp));
+
+  test_offsets_partial offsets[] = {
+      {2, 0, 0, 0}, // no space for first CP
+      {1, 1, 0, 0}, // incomplete first CP
+      {1, 0, 0, 0}, // incomplete first CP, and no space for it
+
+      {4, 1, 2, 1}, // no space for second CP
+      {3, 2, 2, 1}, // incomplete second CP
+      {3, 1, 2, 1}, // incomplete second CP, and no space for it
+
+      {6, 2, 4, 2}, // no space for third CP
+      {5, 3, 4, 2}, // incomplete third CP
+      {5, 2, 4, 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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+  }
+}
+
+template <class InternT>
+void utf16_to_utf32_in_error(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  char16_t input[]          = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const char32_t expected[] = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 5, "");
+
+  InternT exp[array_size(expected)];
+  copy(begin(expected), end(expected), begin(exp));
+
+  // The only possible error in UTF-16 is unpaired surrogate code units.
+  // So we replace valid code points (scalar values) with lone surrogate CU.
+  test_offsets_error<char16_t> offsets[] = {
+      {10, 4, 0, 0, 0xD800, 0},
+      {10, 4, 0, 0, 0xDBFF, 0},
+      {10, 4, 0, 0, 0xDC00, 0},
+      {10, 4, 0, 0, 0xDFFF, 0},
+
+      {10, 4, 2, 1, 0xD800, 1},
+      {10, 4, 2, 1, 0xDBFF, 1},
+      {10, 4, 2, 1, 0xDC00, 1},
+      {10, 4, 2, 1, 0xDFFF, 1},
+
+      {10, 4, 4, 2, 0xD800, 2},
+      {10, 4, 4, 2, 0xDBFF, 2},
+      {10, 4, 4, 2, 0xDC00, 2},
+      {10, 4, 4, 2, 0xDFFF, 2},
+
+      // make the leading surrogate a trailing one
+      {10, 4, 6, 3, 0xDC00, 3},
+      {10, 4, 6, 3, 0xDFFF, 3},
+
+      // make the trailing surrogate a leading one
+      {10, 4, 6, 3, 0xD800, 4},
+      {10, 4, 6, 3, 0xDBFF, 4},
+
+      // make the trailing surrogate a BMP char
+      {10, 4, 6, 3, 'z', 4},
+  };
+
+  for (test_offsets_error<char16_t>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<char16_t> t = *it;
+    char in[array_size(input) * 2];
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    char16_t old_char    = input[t.replace_pos];
+    input[t.replace_pos] = t.replace_char; // replace in input, not in in
+    utf16_to_bytes(begin(input), end(input), begin(in), endianess);
+
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+
+    input[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT>
+void utf32_to_utf16_out_ok(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char32_t input[]    = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 5, "");
+  static_assert(array_size(expected) == 6, "");
+
+  InternT in[array_size(input)];
+  char exp[array_size(expected) * 2];
+  copy(begin(input), end(input), begin(in));
+  utf16_to_bytes(begin(expected), end(expected), begin(exp), endianess);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 2}, {2, 4}, {3, 6}, {4, 10}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t             = *it;
+    char out[array_size(exp) - 2] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    char* out_next           = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<char>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+  }
+}
+
+template <class InternT>
+void utf32_to_utf16_out_partial(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char32_t input[]    = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 5, "");
+  static_assert(array_size(expected) == 6, "");
+
+  InternT in[array_size(input)];
+  char exp[array_size(expected) * 2];
+  copy(begin(input), end(input), begin(in));
+  utf16_to_bytes(begin(expected), end(expected), begin(exp), endianess);
+
+  test_offsets_partial offsets[] = {
+      {1, 0, 0, 0}, // no space for first CP
+      {1, 1, 0, 0}, // no space for first CP
+
+      {2, 2, 1, 2}, // no space for second CP
+      {2, 3, 1, 2}, // no space for second CP
+
+      {3, 4, 2, 4}, // no space for third CP
+      {3, 5, 2, 4}, // 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 (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t        = *it;
+    char out[array_size(exp) - 2] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    char* out_next           = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<char>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+  }
+}
+
+template <class InternT>
+void utf32_to_utf16_out_error(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char32_t input[]    = {'b', 0x0448, 0xAAAA, 0x10AAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 5, "");
+  static_assert(array_size(expected) == 6, "");
+
+  InternT in[array_size(input)];
+  char exp[array_size(expected) * 2];
+  copy(begin(input), end(input), begin(in));
+  utf16_to_bytes(begin(expected), end(expected), begin(exp), endianess);
+
+  test_offsets_error<InternT> offsets[] = {
+
+      // Surrogate CP
+      {4, 10, 0, 0, 0xD800, 0},
+      {4, 10, 1, 2, 0xDBFF, 1},
+      {4, 10, 2, 4, 0xDC00, 2},
+      {4, 10, 3, 6, 0xDFFF, 3},
+
+      // CP out of range
+      {4, 10, 0, 0, 0x00110000, 0},
+      {4, 10, 1, 2, 0x00110000, 1},
+      {4, 10, 2, 4, 0x00110000, 2},
+      {4, 10, 3, 6, 0x00110000, 3}};
+
+  for (test_offsets_error<InternT>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<InternT> t = *it;
+    char out[array_size(exp) - 2] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    InternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    char* out_next           = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<char>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT>
+void test_utf16_utf32_cvt(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  utf16_to_utf32_in_ok(cvt, endianess);
+  utf16_to_utf32_in_partial(cvt, endianess);
+  utf16_to_utf32_in_error(cvt, endianess);
+  utf32_to_utf16_out_ok(cvt, endianess);
+  utf32_to_utf16_out_partial(cvt, endianess);
+  utf32_to_utf16_out_error(cvt, endianess);
+}
+
+template <class InternT>
+void utf16_to_ucs2_in_ok(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0};
+  static_assert(array_size(input) == 4, "");
+  static_assert(array_size(expected) == 4, "");
+
+  char in[array_size(input) * 2];
+  InternT exp[array_size(expected)];
+  utf16_to_bytes(begin(input), end(input), begin(in), endianess);
+  copy(begin(expected), end(expected), begin(exp));
+
+  test_offsets_ok offsets[] = {{0, 0}, {2, 1}, {4, 2}, {6, 3}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t                = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t            = *it;
+    InternT out[array_size(exp)] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, end(out), out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<InternT>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, array_size(out));
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.in_size);
+  }
+}
+
+template <class InternT>
+void utf16_to_ucs2_in_partial(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0};
+  static_assert(array_size(input) == 4, "");
+  static_assert(array_size(expected) == 4, "");
+
+  char in[array_size(input) * 2];
+  InternT exp[array_size(expected)];
+  utf16_to_bytes(begin(input), end(input), begin(in), endianess);
+  copy(begin(expected), end(expected), begin(exp));
+
+  test_offsets_partial offsets[] = {
+      {2, 0, 0, 0}, // no space for first CP
+      {1, 1, 0, 0}, // incomplete first CP
+      {1, 0, 0, 0}, // incomplete first CP, and no space for it
+
+      {4, 1, 2, 1}, // no space for second CP
+      {3, 2, 2, 1}, // incomplete second CP
+      {3, 1, 2, 1}, // incomplete second CP, and no space for it
+
+      {6, 2, 4, 2}, // no space for third CP
+      {5, 3, 4, 2}, // incomplete third CP
+      {5, 2, 4, 2}, // incomplete third CP, and no space for it
+  };
+
+  for (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t           = *it;
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+  }
+}
+
+template <class InternT>
+void utf16_to_ucs2_in_error(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  char16_t input[]          = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 6, "");
+
+  InternT exp[array_size(expected)];
+  copy(begin(expected), end(expected), begin(exp));
+
+  // The only possible error in UTF-16 is unpaired surrogate code units.
+  // Additionally, because the target encoding is UCS-2, a proper pair of
+  // surrogates is also error. Simply, any surrogate CU is error.
+  test_offsets_error<char16_t> offsets[] = {
+      {6, 3, 0, 0, 0xD800, 0},
+      {6, 3, 0, 0, 0xDBFF, 0},
+      {6, 3, 0, 0, 0xDC00, 0},
+      {6, 3, 0, 0, 0xDFFF, 0},
+
+      {6, 3, 2, 1, 0xD800, 1},
+      {6, 3, 2, 1, 0xDBFF, 1},
+      {6, 3, 2, 1, 0xDC00, 1},
+      {6, 3, 2, 1, 0xDFFF, 1},
+
+      {6, 3, 4, 2, 0xD800, 2},
+      {6, 3, 4, 2, 0xDBFF, 2},
+      {6, 3, 4, 2, 0xDC00, 2},
+      {6, 3, 4, 2, 0xDFFF, 2},
+
+      // make the leading surrogate a trailing one
+      {10, 5, 6, 3, 0xDC00, 3},
+      {10, 5, 6, 3, 0xDFFF, 3},
+
+      // make the trailing surrogate a leading one
+      {10, 5, 6, 3, 0xD800, 4},
+      {10, 5, 6, 3, 0xDBFF, 4},
+
+      // make the trailing surrogate a BMP char
+      {10, 5, 6, 3, 'z', 4},
+
+      // don't replace anything in the test cases bellow, just show the surrogate
+      // pair (fourth CP) fully or partially (just the first surrogate)
+      {10, 5, 6, 3, 'b', 0},
+      {8, 5, 6, 3, 'b', 0},
+      {9, 5, 6, 3, 'b', 0},
+
+      {10, 4, 6, 3, 'b', 0},
+      {8, 4, 6, 3, 'b', 0},
+      {9, 4, 6, 3, 'b', 0},
+  };
+
+  for (test_offsets_error<char16_t>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<char16_t> t = *it;
+    char in[array_size(input) * 2];
+    InternT out[array_size(exp) - 1] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    char16_t old_char    = input[t.replace_pos];
+    input[t.replace_pos] = t.replace_char; // replace in input, not in in
+    utf16_to_bytes(begin(input), end(input), begin(in), endianess);
+
+    mbstate_t state          = {};
+    const char* in_next      = nullptr;
+    InternT* out_next        = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.in(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<InternT>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    state   = mbstate_t();
+    int len = cvt.length(state, in, in + t.in_size, t.out_size);
+    assert(len >= 0);
+    assert(static_cast<size_t>(len) == t.expected_in_next);
+
+    input[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT>
+void ucs2_to_utf16_out_ok(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0};
+  static_assert(array_size(input) == 4, "");
+  static_assert(array_size(expected) == 4, "");
+
+  InternT in[array_size(input)];
+  char exp[array_size(expected) * 2];
+  copy(begin(input), end(input), begin(in));
+  utf16_to_bytes(begin(expected), end(expected), begin(exp), endianess);
+
+  test_offsets_ok offsets[] = {{0, 0}, {1, 2}, {2, 4}, {3, 6}};
+  for (test_offsets_ok* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_ok t             = *it;
+    char out[array_size(exp) - 2] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    char* out_next           = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.ok);
+    assert(in_next == in + t.in_size);
+    assert(out_next == out + t.out_size);
+    assert(char_traits<char>::compare(out, exp, t.out_size) == 0);
+    if (t.out_size < array_size(out))
+      assert(out[t.out_size] == 0);
+  }
+}
+
+template <class InternT>
+void ucs2_to_utf16_out_partial(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0};
+  static_assert(array_size(input) == 4, "");
+  static_assert(array_size(expected) == 4, "");
+
+  InternT in[array_size(input)];
+  char exp[array_size(expected) * 2];
+  copy(begin(input), end(input), begin(in));
+  utf16_to_bytes(begin(expected), end(expected), begin(exp), endianess);
+
+  test_offsets_partial offsets[] = {
+      {1, 0, 0, 0}, // no space for first CP
+      {1, 1, 0, 0}, // no space for first CP
+
+      {2, 2, 1, 2}, // no space for second CP
+      {2, 3, 1, 2}, // no space for second CP
+
+      {3, 4, 2, 4}, // no space for third CP
+      {3, 5, 2, 4}, // no space for third CP
+  };
+  for (test_offsets_partial* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_partial t        = *it;
+    char out[array_size(exp) - 2] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    char* out_next           = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.partial);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<char>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+  }
+}
+
+template <class InternT>
+void ucs2_to_utf16_out_error(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  const char16_t input[]    = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  const char16_t expected[] = {'b', 0x0448, 0xAAAA, 0xDBEA, 0xDEAA, 0};
+  static_assert(array_size(input) == 6, "");
+  static_assert(array_size(expected) == 6, "");
+
+  InternT in[array_size(input)];
+  char exp[array_size(expected) * 2];
+  copy(begin(input), end(input), begin(in));
+  utf16_to_bytes(begin(expected), end(expected), begin(exp), endianess);
+
+  test_offsets_error<InternT> offsets[] = {
+      {3, 6, 0, 0, 0xD800, 0},
+      {3, 6, 0, 0, 0xDBFF, 0},
+      {3, 6, 0, 0, 0xDC00, 0},
+      {3, 6, 0, 0, 0xDFFF, 0},
+
+      {3, 6, 1, 2, 0xD800, 1},
+      {3, 6, 1, 2, 0xDBFF, 1},
+      {3, 6, 1, 2, 0xDC00, 1},
+      {3, 6, 1, 2, 0xDFFF, 1},
+
+      {3, 6, 2, 4, 0xD800, 2},
+      {3, 6, 2, 4, 0xDBFF, 2},
+      {3, 6, 2, 4, 0xDC00, 2},
+      {3, 6, 2, 4, 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, 'z', 4},
+
+      // don't replace anything in the test cases bellow, just show the surrogate
+      // pair (fourth CP) fully or partially (just the first surrogate)
+      {5, 10, 3, 6, 'b', 0},
+      {5, 8, 3, 6, 'b', 0},
+      {5, 9, 3, 6, 'b', 0},
+
+      {4, 10, 3, 6, 'b', 0},
+      {4, 8, 3, 6, 'b', 0},
+      {4, 9, 3, 6, 'b', 0},
+  };
+
+  for (test_offsets_error<InternT>* it = begin(offsets); it != end(offsets); ++it) {
+    test_offsets_error<InternT> t = *it;
+    char out[array_size(exp) - 2] = {};
+    assert(t.in_size <= array_size(in));
+    assert(t.out_size <= array_size(out));
+    assert(t.expected_in_next <= t.in_size);
+    assert(t.expected_out_next <= t.out_size);
+    InternT old_char  = in[t.replace_pos];
+    in[t.replace_pos] = t.replace_char;
+
+    mbstate_t state          = {};
+    const InternT* in_next   = nullptr;
+    char* out_next           = nullptr;
+    codecvt_base::result res = codecvt_base::ok;
+
+    res = cvt.out(state, in, in + t.in_size, in_next, out, out + t.out_size, out_next);
+    assert(res == cvt.error);
+    assert(in_next == in + t.expected_in_next);
+    assert(out_next == out + t.expected_out_next);
+    assert(char_traits<char>::compare(out, exp, t.expected_out_next) == 0);
+    if (t.expected_out_next < array_size(out))
+      assert(out[t.expected_out_next] == 0);
+
+    in[t.replace_pos] = old_char;
+  }
+}
+
+template <class InternT>
+void test_utf16_ucs2_cvt(const std::codecvt<InternT, char, mbstate_t>& cvt, utf16_endianess endianess) {
+  utf16_to_ucs2_in_ok(cvt, endianess);
+  utf16_to_ucs2_in_partial(cvt, endianess);
+  utf16_to_ucs2_in_error(cvt, endianess);
+  ucs2_to_utf16_out_ok(cvt, endianess);
+  ucs2_to_utf16_out_partial(cvt, endianess);
+  ucs2_to_utf16_out_error(cvt, endianess);
+}
+
+using std::codecvt;
+using std::codecvt_utf16;
+using std::codecvt_utf8;
+using std::codecvt_utf8_utf16;
+using std::has_facet;
+using std::locale;
+using std::use_facet;
+
+void test_utf8_utf32_codecvts() {
+  typedef codecvt<char32_t, char, mbstate_t> codecvt_c32;
+  const locale& loc_c = locale::classic();
+  assert(has_facet<codecvt_c32>(loc_c));
+
+  const codecvt_c32& cvt = use_facet<codecvt_c32>(loc_c);
+  test_utf8_utf32_cvt(cvt);
+
+  codecvt_utf8<char32_t> cvt2;
+  test_utf8_utf32_cvt(cvt2);
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && !defined(TEST_SHORT_WCHAR)
+  codecvt_utf8<wchar_t> cvt3;
+  test_utf8_utf32_cvt(cvt3);
+#endif
+
+#ifndef TEST_HAS_NO_CHAR8_T
+  typedef codecvt<char32_t, char8_t, mbstate_t> codecvt_c32_c8;
+  assert(has_facet<codecvt_c32_c8>(loc_c));
+  const codecvt_c32_c8& cvt4 = use_facet<codecvt_c32_c8>(loc_c);
+  test_utf8_utf32_cvt(cvt4);
+#endif
+}
+
+void test_utf8_utf16_codecvts() {
+  typedef codecvt<char16_t, char, mbstate_t> codecvt_c16;
+  const locale& loc_c = locale::classic();
+  assert(has_facet<codecvt_c16>(loc_c));
+
+  const codecvt_c16& cvt = use_facet<codecvt_c16>(loc_c);
+  test_utf8_utf16_cvt(cvt);
+
+  codecvt_utf8_utf16<char16_t> cvt2;
+  test_utf8_utf16_cvt(cvt2);
+
+  codecvt_utf8_utf16<char32_t> cvt3;
+  test_utf8_utf16_cvt(cvt3);
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  codecvt_utf8_utf16<wchar_t> cvt4;
+  test_utf8_utf16_cvt(cvt4);
+#endif
+
+#ifndef TEST_HAS_NO_CHAR8_T
+  typedef codecvt<char16_t, char8_t, mbstate_t> codecvt_c16_c8;
+  assert(has_facet<codecvt_c16_c8>(loc_c));
+  const codecvt_c16_c8& cvt5 = use_facet<codecvt_c16_c8>(loc_c);
+  test_utf8_utf16_cvt(cvt5);
+#endif
+}
+
+void test_utf8_ucs2_codecvts() {
+  codecvt_utf8<char16_t> cvt;
+  test_utf8_ucs2_cvt(cvt);
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && defined(TEST_SHORT_WCHAR)
+  codecvt_utf8<wchar_t> cvt2;
+  test_utf8_ucs2_cvt(cvt2);
+#endif
+}
+
+void test_utf16_utf32_codecvts() {
+  codecvt_utf16<char32_t> cvt;
+  test_utf16_utf32_cvt(cvt, utf16_big_endian);
+
+  codecvt_utf16<char32_t, 0x10FFFF, std::little_endian> cvt2;
+  test_utf16_utf32_cvt(cvt2, utf16_little_endian);
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && !defined(TEST_SHORT_WCHAR)
+  codecvt_utf16<wchar_t> cvt3;
+  test_utf16_utf32_cvt(cvt3, utf16_big_endian);
+
+  codecvt_utf16<wchar_t, 0x10FFFF, std::little_endian> cvt4;
+  test_utf16_utf32_cvt(cvt4, utf16_little_endian);
+#endif
+}
+
+void test_utf16_ucs2_codecvts() {
+  codecvt_utf16<char16_t> cvt;
+  test_utf16_ucs2_cvt(cvt, utf16_big_endian);
+
+  codecvt_utf16<char16_t, 0x10FFFF, std::little_endian> cvt2;
+  test_utf16_ucs2_cvt(cvt2, utf16_little_endian);
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && defined(TEST_SHORT_WCHAR)
+  codecvt_utf16<wchar_t> cvt3;
+  test_utf16_ucs2_cvt(cvt3, utf16_big_endian);
+
+  codecvt_utf16<wchar_t, 0x10FFFF, std::little_endian> cvt4;
+  test_utf16_ucs2_cvt(cvt4, utf16_little_endian);
+#endif
+}
+
+int main() {
+  test_utf8_utf32_codecvts();
+  test_utf8_utf16_codecvts();
+  test_utf8_ucs2_codecvts();
+  test_utf16_utf32_codecvts();
+  test_utf16_ucs2_codecvts();
+}


        


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