[libc-commits] [libc] [libc] Add UTF-16 support in CharacterConverter (PR #230434)

via libc-commits libc-commits at lists.llvm.org
Fri Oct 9 03:04:07 PDT 2026


https://github.com/yangbowen created https://github.com/llvm/llvm-project/pull/230434

This pull request adds support of UTF-16 in `CharacterConverter`. It does not use the functionality yet. Follow-ups will add uses to the functionality that this PR implements.

Aside from `push(char16_t)` and `pop_utf16()` that implements the conversion, another function `isPartiallyPopping()` is added, which distinguishes if the current state is "partially pushing" or "partially popping". Since UTF-16 can have 2 code units for 1 code point, this reporting function will be crucial to the follow-up uses.
For `isPartiallyPopping()` to work, UTF-8 overlong encodings must be rejected. They pose a security hazard anyway. This PR adds new checks that reject UTF-8 overlong encodings. This PR also adds new checks that reject `>U+10FFFF` inputs.

Tests are added to cover the above-mentioned functionalities. This includes testing that `isPartiallyPopping()` works in UTF-8 -> UTF-32 conversions, which the function is designed to work.

>From 5693e91825a6e2b4f5b596406307d70ead6665df Mon Sep 17 00:00:00 2001
From: yangbowen <ybw1234567 at hotmail.com>
Date: Thu, 8 Oct 2026 20:23:01 +0000
Subject: [PATCH] [libc] Add UTF-16 support in CharacterConverter

---
 libc/hdr/types/CMakeLists.txt                 |  11 +
 libc/hdr/types/char16_t.h                     |  27 ++
 libc/src/__support/wchar/CMakeLists.txt       |   2 +
 .../src/__support/wchar/character_converter.h | 183 +++++++++++++-
 libc/test/src/__support/wchar/CMakeLists.txt  |  26 ++
 .../src/__support/wchar/utf16_to_8_test.cpp   | 235 ++++++++++++++++++
 .../src/__support/wchar/utf32_to_8_test.cpp   |  26 +-
 .../src/__support/wchar/utf8_to_16_test.cpp   | 183 ++++++++++++++
 .../src/__support/wchar/utf8_to_32_test.cpp   | 139 +++++++++++
 9 files changed, 816 insertions(+), 16 deletions(-)
 create mode 100644 libc/hdr/types/char16_t.h
 create mode 100644 libc/test/src/__support/wchar/utf16_to_8_test.cpp
 create mode 100644 libc/test/src/__support/wchar/utf8_to_16_test.cpp

diff --git a/libc/hdr/types/CMakeLists.txt b/libc/hdr/types/CMakeLists.txt
index 92a2985b74be72..6ca38970729cfd 100644
--- a/libc/hdr/types/CMakeLists.txt
+++ b/libc/hdr/types/CMakeLists.txt
@@ -9,6 +9,17 @@ add_proxy_header_library(
     libc.include.uchar
 )
 
+add_proxy_header_library(
+  char16_t
+  HDRS
+    char16_t.h
+  DEPENDS
+    libc.hdr.uchar_overlay
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.char16_t
+    libc.include.uchar
+)
+
 add_proxy_header_library(
   char32_t
   HDRS
diff --git a/libc/hdr/types/char16_t.h b/libc/hdr/types/char16_t.h
new file mode 100644
index 00000000000000..a59d698243b090
--- /dev/null
+++ b/libc/hdr/types/char16_t.h
@@ -0,0 +1,27 @@
+//===-- Definition of char16_t.h ------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_TYPES_CHAR16_T_H
+#define LLVM_LIBC_HDR_TYPES_CHAR16_T_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/char16_t.h"
+
+#else // overlay mode
+
+// MacOS doesn't provide uchar.h so we use the types provided by LLVM-libc.
+#ifdef __APPLE__
+#include "include/llvm-libc-types/char16_t.h"
+#else
+#include "hdr/uchar_overlay.h"
+#endif
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_CHAR16_T_H
diff --git a/libc/src/__support/wchar/CMakeLists.txt b/libc/src/__support/wchar/CMakeLists.txt
index c4e7f0dbaa1786..91fd788fd07627 100644
--- a/libc/src/__support/wchar/CMakeLists.txt
+++ b/libc/src/__support/wchar/CMakeLists.txt
@@ -29,11 +29,13 @@ add_header_library(
   DEPENDS
     libc.hdr.errno_macros
     libc.hdr.types.char8_t
+    libc.hdr.types.char16_t
     libc.hdr.types.char32_t
     libc.hdr.types.size_t
     libc.src.__support.common
     libc.src.__support.CPP.type_traits
     libc.src.__support.error_or
+    libc.src.__support.libc_assert
     libc.src.__support.macros.config
     libc.src.__support.macros.properties.types
     libc.src.__support.math_extras
diff --git a/libc/src/__support/wchar/character_converter.h b/libc/src/__support/wchar/character_converter.h
index f754a602d2cce0..f7ccf6f18b36cc 100644
--- a/libc/src/__support/wchar/character_converter.h
+++ b/libc/src/__support/wchar/character_converter.h
@@ -10,6 +10,7 @@
 #define LLVM_LIBC_SRC___SUPPORT_CHARACTER_CONVERTER_H
 
 #include "hdr/errno_macros.h"
+#include "hdr/types/char16_t.h"
 #include "hdr/types/char32_t.h"
 #include "hdr/types/char8_t.h"
 #include "hdr/types/size_t.h"
@@ -18,6 +19,7 @@
 #include "src/__support/CPP/type_traits.h"
 #include "src/__support/common.h"
 #include "src/__support/error_or.h"
+#include "src/__support/libc_assert.h"
 #include "src/__support/macros/properties/types.h"
 #include "src/__support/math_extras.h"
 #include "src/__support/wchar/mbstate.h"
@@ -57,18 +59,65 @@ class CharacterConverter {
   LIBC_INLINE bool isEmpty() { return state->bytes_stored == 0; }
   bool isValidState();
 
+  /*
+   * When converting between UTF-16 and UTF-8, the converter might be in a state
+   * of either partially-pushing or partially-popping. For example consider the
+   * following sequence:
+   * *   push((char8_t)0xF0);
+   * *   push((char8_t)0xA4);
+   * *   [A](partial=0x24,bytes_stored=2,total_bytes=4)
+   * *   push((char8_t)0xAD);
+   * *   push((char8_t)0xA2);
+   * *   (partial=0x24B62,bytes_stored=4,total_bytes=4)
+   * *   pop_utf16(); -> 0xD852
+   * *   [B](partial=0x24B62,bytes_stored=2,total_bytes=4)
+   * *   pop_utf16(); -> 0xDF62
+   * At point [A] and point [B], the stored bytes would both be 2. But at [A] we
+   * must continue reading input, while at [B] we must write the output low
+   * surrogate and clear the state.
+   * Thus we must have some means to distinguish between pushing and popping.
+   * Notice that: When pushing, we do left-shifts on state->partial before each
+   * push. When popping, we keep state->partial at the complete code point.
+   * Since we disallow overlong encodings, the complete UTF-8 code point is
+   * always greater than
+   * *   MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 2]
+   * but at most
+   * *   MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 1]
+   * If we are partially through pushing, state->partial is at most
+   * *   char32 >> 6
+   * thus at most
+   * *   MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 1] >> 6
+   * Observe that, an item in MAX_VALUE_PER_UTF8_LEN right-shifted by 6 is
+   * always less than the preceding item. Based on these facts, we can determine
+   * if we are partially through pushing or popping, by comparing state->partial
+   * with MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 2]
+   * Also when converting UTF-16 -> UTF-8, in a partial state,
+   * *   bytes_stored=2
+   * *   total_bytes=4
+   * so the threshold 0xFFFF is also correct.
+   */
+  LIBC_INLINE bool isPartiallyPopping() {
+    if (isEmpty() || isFull())
+      return false;
+    // When neither empty nor full, state->total_bytes is at least 2
+    char32_t threshold = MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 2];
+    return state->partial > threshold;
+  }
+
   template <typename CharType> size_t sizeAs();
 
   int push(char8_t utf8_byte);
+  int push(char16_t utf16);
   int push(char32_t utf32);
 
   ErrorOr<char8_t> pop_utf8();
+  ErrorOr<char16_t> pop_utf16();
   ErrorOr<char32_t> pop_utf32();
 
-#if defined(LIBC_TYPES_WCHAR_T_IS_UTF32)
+#if defined(LIBC_TYPES_WCHAR_T_IS_UTF32) || defined(LIBC_TYPES_WCHAR_T_IS_UTF16)
   int push(wchar_t wchar);
   ErrorOr<wchar_t> pop_wchar();
-#endif // LIBC_TYPES_WCHAR_T_IS_UTF32
+#endif // LIBC_TYPES_WCHAR_T_IS_UTF32 || LIBC_TYPES_WCHAR_T_IS_UTF16
 
   template <typename CharType> ErrorOr<CharType> pop();
 };
@@ -112,6 +161,11 @@ LIBC_INLINE int CharacterConverter::push(char8_t utf8_byte) {
     state->bytes_stored++;
     return 0;
   }
+  // Pushing when partially popping is a misuse in the library
+  if (isPartiallyPopping()) {
+    clear();
+    return -1;
+  }
   // Any subsequent push
   // Adding 6 more bits so need to left shift
   if (num_ones == 1 && !isFull()) {
@@ -119,6 +173,19 @@ LIBC_INLINE int CharacterConverter::push(char8_t utf8_byte) {
     state->partial = state->partial << ENCODED_BITS_PER_UTF8;
     state->partial |= byte;
     state->bytes_stored++;
+    if (isFull()) {
+      // Detect and reject overlong encodings
+      // Needed for isPartiallyPopping to work
+      if (state->partial <= MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 2]) {
+        clear();
+        return EILSEQ;
+      }
+      // Also reject out-of-range code points
+      if (state->partial > MAX_VALUE_PER_UTF8_LEN[state->total_bytes - 1]) {
+        clear();
+        return EILSEQ;
+      }
+    }
     return 0;
   }
 
@@ -127,6 +194,57 @@ LIBC_INLINE int CharacterConverter::push(char8_t utf8_byte) {
   return EILSEQ;
 }
 
+LIBC_INLINE int CharacterConverter::push(char16_t utf16) {
+  // Checking for high surrogate if first push
+  if (isEmpty()) {
+    bool is_surrogate = (utf16 & 0xF800) == 0xD800;
+    if (is_surrogate) {
+      bool is_high_surrogate = (utf16 & 0x400) == 0;
+      if (!is_high_surrogate) {
+        // unpaired surrogate -> reset the state
+        // bytes_stored and total_bytes will always be 0 here
+        state->partial = static_cast<char32_t>(0);
+        return EILSEQ;
+      }
+      // high surrogate
+      // code point between 0x10000 and 0x10FFFF
+      state->total_bytes = 4;
+      state->bytes_stored = 2;
+      // 10 bits each from the high surrogate and the low surrogate, and the
+      // full code point is from 0x10000, so we add 0x40 to partial, which after
+      // shifting becomes 0x10000
+      state->partial = static_cast<char32_t>(utf16 & 0x3FF) + 0x40;
+    } else {
+      state->partial = static_cast<char32_t>(utf16);
+      // determine number of utf-8 bytes needed to represent this utf16 value
+      uint8_t i;
+      for (i = 0; i < 2; i++) {
+        if (state->partial <= MAX_VALUE_PER_UTF8_LEN[i])
+          break;
+      }
+      state->total_bytes = i + 1;
+      state->bytes_stored = i + 1;
+    }
+    return 0;
+  }
+  // Pushing when partially popping is a misuse in the library
+  if (isFull() || isPartiallyPopping()) {
+    clear();
+    return -1;
+  }
+  bool is_low_surrogate = (utf16 & 0xFC00) == 0xDC00;
+  if (!is_low_surrogate) {
+    // unpaired surrogate -> reset the state
+    clear();
+    return EILSEQ;
+  }
+  char32_t low = utf16 & 0x3FF;
+  state->partial <<= 10;
+  state->partial |= low;
+  state->bytes_stored += 2;
+  return 0;
+}
+
 LIBC_INLINE int CharacterConverter::push(char32_t utf32) {
   // we can't be partially through a conversion when pushing a utf32 value
   if (!isEmpty())
@@ -160,8 +278,36 @@ LIBC_INLINE ErrorOr<char32_t> CharacterConverter::pop_utf32() {
   return utf32;
 }
 
+LIBC_INLINE ErrorOr<char16_t> CharacterConverter::pop_utf16() {
+  if (!isFull() && !isPartiallyPopping())
+    return Error(-1);
+  char32_t output;
+  if (state->total_bytes <= 3) {
+    // no surrogates needed
+    output = state->partial;
+    // disallow using UTF-8 or UTF-32 to encode unpaired surrogates
+    bool is_surrogate = (output & 0xF800) == 0xD800;
+    if (is_surrogate) {
+      clear();
+      return Error(EILSEQ);
+    }
+    clear();
+  } else {
+    if (isFull()) {
+      // high surrogate first
+      output = ((state->partial - 0x10000) >> 10) | 0xD800;
+      state->bytes_stored = 2;
+    } else {
+      // low surrogate
+      output = ((state->partial - 0x10000) & 0x3FF) | 0xDC00;
+      clear();
+    }
+  }
+  return static_cast<char16_t>(output);
+}
+
 LIBC_INLINE ErrorOr<char8_t> CharacterConverter::pop_utf8() {
-  if (isEmpty())
+  if (!isFull() && !isPartiallyPopping())
     return Error(-1);
 
   constexpr char8_t FIRST_BYTE_HEADERS[] = {0, 0xC0, 0xE0, 0xF0};
@@ -196,6 +342,10 @@ template <> LIBC_INLINE ErrorOr<char8_t> CharacterConverter::pop() {
   return pop_utf8();
 }
 
+template <> LIBC_INLINE ErrorOr<char16_t> CharacterConverter::pop() {
+  return pop_utf16();
+}
+
 template <> LIBC_INLINE ErrorOr<char32_t> CharacterConverter::pop() {
   return pop_utf32();
 }
@@ -204,6 +354,10 @@ template <> LIBC_INLINE size_t CharacterConverter::sizeAs<char8_t>() {
   return state->total_bytes;
 }
 
+template <> LIBC_INLINE size_t CharacterConverter::sizeAs<char16_t>() {
+  return state->total_bytes > 3 ? 2 : 1;
+}
+
 template <> LIBC_INLINE size_t CharacterConverter::sizeAs<char32_t>() {
   return 1;
 }
@@ -229,7 +383,28 @@ template <> LIBC_INLINE size_t CharacterConverter::sizeAs<wchar_t>() {
   return sizeAs<char32_t>();
 }
 
-#endif // LIBC_TYPES_WCHAR_T_IS_UTF32
+#elif defined(LIBC_TYPES_WCHAR_T_IS_UTF16)
+
+LIBC_INLINE int CharacterConverter::push(wchar_t wchar) {
+  return push(static_cast<char16_t>(wchar));
+}
+
+LIBC_INLINE ErrorOr<wchar_t> CharacterConverter::pop_wchar() {
+  ErrorOr<char16_t> Result = pop_utf16();
+  if (!Result)
+    return Error(Result.error());
+  return static_cast<wchar_t>(*Result);
+}
+
+template <> LIBC_INLINE ErrorOr<wchar_t> CharacterConverter::pop() {
+  return pop_wchar();
+}
+
+template <> LIBC_INLINE size_t CharacterConverter::sizeAs<wchar_t>() {
+  return sizeAs<char16_t>();
+}
+
+#endif // LIBC_TYPES_WCHAR_T_IS_UTF32 || LIBC_TYPES_WCHAR_T_IS_UTF16
 
 } // namespace internal
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/src/__support/wchar/CMakeLists.txt b/libc/test/src/__support/wchar/CMakeLists.txt
index 259e896ee835d4..907950833dbcc0 100644
--- a/libc/test/src/__support/wchar/CMakeLists.txt
+++ b/libc/test/src/__support/wchar/CMakeLists.txt
@@ -41,6 +41,32 @@ add_libc_test(
     libc.src.__support.wchar.mbstate
 )
 
+add_libc_test(
+  utf8_to_16_test
+  SUITE
+    libc-support-tests
+  SRCS
+    utf8_to_16_test.cpp
+  DEPENDS
+    libc.src.__support.error_or
+    libc.src.__support.macros.properties.types
+    libc.src.__support.wchar.character_converter
+    libc.src.__support.wchar.mbstate
+)
+
+add_libc_test(
+  utf16_to_8_test
+  SUITE
+    libc-support-tests
+  SRCS
+    utf16_to_8_test.cpp
+  DEPENDS
+    libc.src.__support.error_or
+    libc.src.__support.macros.properties.types
+    libc.src.__support.wchar.character_converter
+    libc.src.__support.wchar.mbstate
+)
+
 add_libc_test(
   wcsnrtombs_test
   SUITE
diff --git a/libc/test/src/__support/wchar/utf16_to_8_test.cpp b/libc/test/src/__support/wchar/utf16_to_8_test.cpp
new file mode 100644
index 00000000000000..737f17595ce107
--- /dev/null
+++ b/libc/test/src/__support/wchar/utf16_to_8_test.cpp
@@ -0,0 +1,235 @@
+//===-- Unittests for the CharacterConverter class (utf16 -> 8) -----------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/common.h"
+#include "src/__support/macros/properties/types.h"
+#include "src/__support/wchar/character_converter.h"
+#include "src/__support/wchar/mbstate.h"
+
+#include "test/UnitTest/Test.h"
+
+#if defined(LIBC_TYPES_WCHAR_T_IS_UTF16)
+using TestCharTypesUTF16 = LIBC_NAMESPACE::testing::TypeList<char16_t, wchar_t>;
+#else
+using TestCharTypesUTF16 = LIBC_NAMESPACE::testing::TypeList<char16_t>;
+#endif
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF16To8Test, OneByte,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  LIBC_NAMESPACE::internal::CharacterConverter cr(&state);
+  cr.clear();
+
+  // utf8 1-byte encodings are identical to their utf16 representations
+  CharType16 utf16_A = 0x41; // 'A'
+  cr.push(utf16_A);
+  ASSERT_TRUE(cr.isFull());
+  auto popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<char>(popped.value()), 'A');
+  ASSERT_TRUE(cr.isEmpty());
+
+  CharType16 utf16_B = 0x42; // 'B'
+  cr.push(utf16_B);
+  ASSERT_TRUE(cr.isFull());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<char>(popped.value()), 'B');
+  ASSERT_TRUE(cr.isEmpty());
+
+  // should error if we try to pop another utf8 byte out
+  popped = cr.pop_utf8();
+  ASSERT_FALSE(popped.has_value());
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF16To8Test, TwoByte,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  LIBC_NAMESPACE::internal::CharacterConverter cr(&state);
+  cr.clear();
+
+  // testing utf16: 0xff -> utf8: 0xc3 0xbf
+  CharType16 utf16 = 0xff;
+  cr.push(utf16);
+  ASSERT_TRUE(cr.isFull());
+  auto popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xc3);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xbf);
+  ASSERT_TRUE(cr.isEmpty());
+
+  // testing utf16: 0x58e -> utf8: 0xd6 0x8e
+  utf16 = 0x58e;
+  cr.push(utf16);
+  ASSERT_TRUE(cr.isFull());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xd6);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0x8e);
+  ASSERT_TRUE(cr.isEmpty());
+
+  // should error if we try to pop another utf8 byte out
+  popped = cr.pop_utf8();
+  ASSERT_FALSE(popped.has_value());
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF16To8Test, ThreeByte,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  LIBC_NAMESPACE::internal::CharacterConverter cr(&state);
+  cr.clear();
+
+  // testing utf16: 0xac15 -> utf8: 0xea 0xb0 0x95
+  CharType16 utf16 = 0xac15;
+  cr.push(utf16);
+  ASSERT_TRUE(cr.isFull());
+  auto popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xea);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xb0);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0x95);
+  ASSERT_TRUE(cr.isEmpty());
+
+  // testing utf16: 0x267b -> utf8: 0xe2 0x99 0xbb
+  utf16 = 0x267b;
+  cr.push(utf16);
+  ASSERT_TRUE(cr.isFull());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xe2);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0x99);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xbb);
+  ASSERT_TRUE(cr.isEmpty());
+
+  // should error if we try to pop another utf8 byte out
+  popped = cr.pop_utf8();
+  ASSERT_FALSE(popped.has_value());
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF16To8Test, FourByte,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  LIBC_NAMESPACE::internal::CharacterConverter cr(&state);
+  cr.clear();
+
+  // testing utf16: 0xd83e 0xdd21 -> utf8: 0xf0 0x9f 0xa4 0xa1
+  CharType16 utf16_h = 0xd83e;
+  cr.push(utf16_h);
+  ASSERT_FALSE(cr.isPartiallyPopping());
+  CharType16 utf16_l = 0xdd21;
+  cr.push(utf16_l);
+  ASSERT_TRUE(cr.isFull());
+  auto popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xf0);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0x9f);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xa4);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xa1);
+  ASSERT_TRUE(cr.isEmpty());
+
+  // testing utf16: 0xd808 0xdd21 -> utf8: 0xf0 0x92 0x84 0xa1
+  utf16_h = 0xd808;
+  cr.push(utf16_h);
+  ASSERT_FALSE(cr.isPartiallyPopping());
+  utf16_l = 0xdd21;
+  cr.push(utf16_l);
+  ASSERT_TRUE(cr.isFull());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xf0);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0x92);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0x84);
+  ASSERT_TRUE(cr.isPartiallyPopping());
+  popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+  ASSERT_EQ(static_cast<int>(popped.value()), 0xa1);
+  ASSERT_TRUE(cr.isEmpty());
+
+  // should error if we try to pop another utf8 byte out
+  popped = cr.pop_utf8();
+  ASSERT_FALSE(popped.has_value());
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF16To8Test, CantPushMidConversion,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  LIBC_NAMESPACE::internal::CharacterConverter cr(&state);
+  cr.clear();
+
+  // testing utf16: 0xd808 0xdd21 -> utf8: 0xf0 0x92 0x84 0xa1
+  CharType16 utf16_h = 0xd808;
+  ASSERT_EQ(cr.push(utf16_h), 0);
+  CharType16 utf16_l = 0xdd21;
+  ASSERT_EQ(cr.push(utf16_l), 0);
+  auto popped = cr.pop_utf8();
+  ASSERT_TRUE(popped.has_value());
+
+  // can't push a utf16 without finishing popping the utf8 bytes out
+  int err = cr.push(utf16_l);
+  ASSERT_EQ(err, -1);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF16To8Test, CantPopMidPushing,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  LIBC_NAMESPACE::internal::CharacterConverter cr(&state);
+  cr.clear();
+
+  // testing utf16: 0xd808 0xdd21 -> utf8: 0xf0 0x92 0x84 0xa1
+  CharType16 utf16_h = 0xd808;
+  ASSERT_EQ(cr.push(utf16_h), 0);
+
+  // can't pop a utf8 without finishing pushing the utf16 code units
+  auto popped = cr.pop_utf8();
+  ASSERT_FALSE(popped.has_value());
+}
diff --git a/libc/test/src/__support/wchar/utf32_to_8_test.cpp b/libc/test/src/__support/wchar/utf32_to_8_test.cpp
index 52e10e11911753..bc911050cff353 100644
--- a/libc/test/src/__support/wchar/utf32_to_8_test.cpp
+++ b/libc/test/src/__support/wchar/utf32_to_8_test.cpp
@@ -64,7 +64,7 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF32To8Test, TwoByte,
   auto popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xc3);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xbf);
@@ -77,7 +77,7 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF32To8Test, TwoByte,
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xd6);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0x8e);
@@ -103,11 +103,11 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF32To8Test, ThreeByte,
   auto popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xea);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xb0);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0x95);
@@ -120,11 +120,11 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF32To8Test, ThreeByte,
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xe2);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0x99);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xbb);
@@ -150,15 +150,15 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF32To8Test, FourByte,
   auto popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xf0);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0x9f);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xa4);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xa1);
@@ -171,15 +171,15 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF32To8Test, FourByte,
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xf0);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0x92);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0x84);
-  ASSERT_TRUE(!cr.isEmpty());
+  ASSERT_TRUE(cr.isPartiallyPopping());
   popped = cr.pop_utf8();
   ASSERT_TRUE(popped.has_value());
   ASSERT_EQ(static_cast<int>(popped.value()), 0xa1);
@@ -213,6 +213,8 @@ TEST(LlvmLibcCharacterConverterUTF32To8Test, InvalidState) {
   LIBC_NAMESPACE::internal::mbstate s1;
   LIBC_NAMESPACE::internal::CharacterConverter c1(&s1);
   ASSERT_TRUE(c1.isValidState());
+  ASSERT_TRUE(c1.isEmpty());
+  ASSERT_FALSE(c1.isPartiallyPopping());
 
   LIBC_NAMESPACE::internal::mbstate s2{0, 2, 0};
   LIBC_NAMESPACE::internal::CharacterConverter c2(&s2);
diff --git a/libc/test/src/__support/wchar/utf8_to_16_test.cpp b/libc/test/src/__support/wchar/utf8_to_16_test.cpp
new file mode 100644
index 00000000000000..ed38ddb7596b86
--- /dev/null
+++ b/libc/test/src/__support/wchar/utf8_to_16_test.cpp
@@ -0,0 +1,183 @@
+//===-- Unittests for character_converter utf8->utf16 ---------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/errno_macros.h"
+#include "src/__support/error_or.h"
+#include "src/__support/macros/properties/types.h"
+#include "src/__support/wchar/character_converter.h"
+#include "src/__support/wchar/mbstate.h"
+#include "test/UnitTest/Test.h"
+
+#if defined(LIBC_TYPES_WCHAR_T_IS_UTF16)
+using TestCharTypesUTF16 = LIBC_NAMESPACE::testing::TypeList<char16_t, wchar_t>;
+#else
+using TestCharTypesUTF16 = LIBC_NAMESPACE::testing::TypeList<char16_t>;
+#endif
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, OneByte,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  char ch = 'A';
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  int err = char_conv.push(static_cast<char8_t>(ch));
+  auto wch = char_conv.pop<CharType16>();
+
+  ASSERT_EQ(err, 0);
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 65);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, TwoBytes,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[2] = {static_cast<char>(0xC2),
+                      static_cast<char>(0x8E)}; // Ž car symbol
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
+  char_conv.push(static_cast<char8_t>(ch[1]));
+  auto wch = char_conv.pop<CharType16>();
+
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 142);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, ThreeBytes,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[3] = {static_cast<char>(0xE2), static_cast<char>(0x88),
+                      static_cast<char>(0x91)}; // ∑ sigma symbol
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
+  char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
+  char_conv.push(static_cast<char8_t>(ch[2]));
+  auto wch = char_conv.pop<CharType16>();
+
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 8721);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, FourBytes,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[4] = {static_cast<char>(0xF0), static_cast<char>(0x9F),
+                      static_cast<char>(0xA4),
+                      static_cast<char>(0xA1)}; // 🤡 clown emoji
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
+  char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
+  char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
+  char_conv.push(static_cast<char8_t>(ch[3]));
+  auto wch_h = char_conv.pop<CharType16>();
+  ASSERT_TRUE(char_conv.isPartiallyPopping());
+  auto wch_l = char_conv.pop<CharType16>();
+
+  ASSERT_TRUE(wch_h.has_value());
+  ASSERT_EQ(static_cast<int>(wch_h.value()), 55358);
+  ASSERT_TRUE(wch_l.has_value());
+  ASSERT_EQ(static_cast<int>(wch_l.value()), 56609);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, ValidTwoByteWithExtraRead,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[3] = {static_cast<char>(0xC2), static_cast<char>(0x8E),
+                      static_cast<char>(0x80)};
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, 0);
+  // Should produce an error on 3rd byte
+  err = char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_EQ(err, EILSEQ);
+
+  // Should produce an error since mbstate was reset
+  auto wch = char_conv.pop<CharType16>();
+  ASSERT_FALSE(wch.has_value());
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, TwoValidTwoBytes,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[4] = {static_cast<char>(0xC2), static_cast<char>(0x8E),
+                      static_cast<char>(0xC7), static_cast<char>(0x8C)};
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, 0);
+  auto wch = char_conv.pop<CharType16>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 142);
+
+  // Second two byte character
+  err = char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[3]));
+  ASSERT_EQ(err, 0);
+  wch = char_conv.pop<CharType16>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 460);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To16Test, InvalidPop,
+           TestCharTypesUTF16) {
+  using CharType16 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  const char ch[2] = {static_cast<char>(0xC2), static_cast<char>(0x8E)};
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  auto wch = char_conv.pop<CharType16>();
+  ASSERT_FALSE(
+      wch.has_value()); // Should fail since we have not read enough bytes
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, 0);
+  wch = char_conv.pop<CharType16>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 142);
+}
diff --git a/libc/test/src/__support/wchar/utf8_to_32_test.cpp b/libc/test/src/__support/wchar/utf8_to_32_test.cpp
index db74cddafcbd76..8e72371ca7c067 100644
--- a/libc/test/src/__support/wchar/utf8_to_32_test.cpp
+++ b/libc/test/src/__support/wchar/utf8_to_32_test.cpp
@@ -49,6 +49,7 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, TwoBytes,
 
   LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
   char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
   char_conv.push(static_cast<char8_t>(ch[1]));
   auto wch = char_conv.pop<CharType32>();
 
@@ -68,7 +69,9 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, ThreeBytes,
 
   LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
   char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
   char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
   char_conv.push(static_cast<char8_t>(ch[2]));
   auto wch = char_conv.pop<CharType32>();
 
@@ -89,8 +92,11 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, FourBytes,
 
   LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
   char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
   char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
   char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_FALSE(char_conv.isPartiallyPopping());
   char_conv.push(static_cast<char8_t>(ch[3]));
   auto wch = char_conv.pop<CharType32>();
 
@@ -223,3 +229,136 @@ TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, InvalidPop,
   ASSERT_TRUE(wch.has_value());
   ASSERT_EQ(static_cast<int>(wch.value()), 142);
 }
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, RejectOverlongTwoBytes,
+           TestCharTypesUTF32) {
+  using CharType32 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[4] = {static_cast<char>(0xC1), static_cast<char>(0xBF),
+                      static_cast<char>(0xC2), static_cast<char>(0x80)};
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  // Overlong encoding should be rejected U+007F
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, EILSEQ);
+
+  // Not overlong, don't reject U+0080
+  err = char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[3]));
+  ASSERT_EQ(err, 0);
+  auto wch = char_conv.pop<CharType32>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 128);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, RejectOverlongThreeBytes,
+           TestCharTypesUTF32) {
+  using CharType32 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[6] = {static_cast<char>(0xE0), static_cast<char>(0x9F),
+                      static_cast<char>(0xBF), static_cast<char>(0xE0),
+                      static_cast<char>(0xA0), static_cast<char>(0x80)};
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  // Overlong encoding should be rejected U+07FF
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_EQ(err, EILSEQ);
+
+  // Not overlong, don't reject U+0800
+  err = char_conv.push(static_cast<char8_t>(ch[3]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[4]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[5]));
+  ASSERT_EQ(err, 0);
+  auto wch = char_conv.pop<CharType32>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 2048);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, RejectOverlongFourBytes,
+           TestCharTypesUTF32) {
+  using CharType32 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[8] = {static_cast<char>(0xF0), static_cast<char>(0x8F),
+                      static_cast<char>(0xBF), static_cast<char>(0xBF),
+                      static_cast<char>(0xF0), static_cast<char>(0x90),
+                      static_cast<char>(0x80), static_cast<char>(0x80)};
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  // Overlong encoding should be rejected U+FFFF
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[3]));
+  ASSERT_EQ(err, EILSEQ);
+
+  // Not overlong, don't reject U+10000
+  err = char_conv.push(static_cast<char8_t>(ch[4]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[5]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[6]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[7]));
+  ASSERT_EQ(err, 0);
+  auto wch = char_conv.pop<CharType32>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 65536);
+}
+
+TYPED_TEST(LlvmLibcCharacterConverterUTF8To32Test, RejectOutOfRange,
+           TestCharTypesUTF32) {
+  using CharType32 = ParamType;
+
+  LIBC_NAMESPACE::internal::mbstate state;
+  state.bytes_stored = 0;
+  state.total_bytes = 0;
+  const char ch[8] = {static_cast<char>(0xF4), static_cast<char>(0x90),
+                      static_cast<char>(0x80), static_cast<char>(0x80),
+                      static_cast<char>(0xF4), static_cast<char>(0x8F),
+                      static_cast<char>(0xBF), static_cast<char>(0xBF)};
+
+  LIBC_NAMESPACE::internal::CharacterConverter char_conv(&state);
+  // Out of range code point should be rejected U+110000
+  int err = char_conv.push(static_cast<char8_t>(ch[0]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[1]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[2]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[3]));
+  ASSERT_EQ(err, EILSEQ);
+
+  // In range, don't reject U+10FFFF
+  err = char_conv.push(static_cast<char8_t>(ch[4]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[5]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[6]));
+  ASSERT_EQ(err, 0);
+  err = char_conv.push(static_cast<char8_t>(ch[7]));
+  ASSERT_EQ(err, 0);
+  auto wch = char_conv.pop<CharType32>();
+  ASSERT_TRUE(wch.has_value());
+  ASSERT_EQ(static_cast<int>(wch.value()), 1114111);
+}



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