Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -1219,6 +1219,12 @@
 #    define _LIBCPP_PACKED
 #  endif
 
+#  if defined(_LIBCPP_GLIBC_PREREQ)
+#    if _LIBCPP_GLIBC_PREREQ(2, 36)
+#        define _LIBCPP_HAS_C8RTOMB_MBRTOC8
+#    endif
+#  endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP___CONFIG
Index: libcxx/include/cuchar
===================================================================
--- libcxx/include/cuchar
+++ libcxx/include/cuchar
@@ -25,6 +25,8 @@
   mbstate_t
   size_t
 
+size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps);   // C++20
+size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps);                     // C++20
 size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps);
 size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps);
 size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps);
@@ -49,6 +51,8 @@
 using ::mbstate_t _LIBCPP_USING_IF_EXISTS;
 using ::size_t _LIBCPP_USING_IF_EXISTS;
 
+using ::mbrtoc8 _LIBCPP_USING_IF_EXISTS;
+using ::c8rtomb _LIBCPP_USING_IF_EXISTS;
 using ::mbrtoc16 _LIBCPP_USING_IF_EXISTS;
 using ::c16rtomb _LIBCPP_USING_IF_EXISTS;
 using ::mbrtoc32 _LIBCPP_USING_IF_EXISTS;
Index: libcxx/include/uchar.h
===================================================================
--- libcxx/include/uchar.h
+++ libcxx/include/uchar.h
@@ -11,7 +11,7 @@
 #define _LIBCPP_UCHAR_H
 
 /*
-    uchar.h synopsis // since C++11
+    uchar.h synopsis // since C11
 
 Macros:
 
@@ -22,7 +22,12 @@
 
   mbstate_t
   size_t
+  char8_t   // C23, Builtin type in C++20
+  char16_t  // Builtin type in C++11
+  char32_t  // Builtin type in C++11
 
+size_t mbrtoc8(char8_t* pc8, const char* s, size_t n, mbstate_t* ps);     // C23
+size_t c8rtomb(char* s, char8_t c8, mbstate_t* ps);                       // C23
 size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps);
 size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps);
 size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps);
Index: libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
===================================================================
--- libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
+++ libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp
@@ -23,6 +23,11 @@
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
+#if defined(_LIBCPP_HAS_C8RTOMB_MBRTOC8)
+ASSERT_SAME_TYPE(size_t, decltype(mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(size_t, decltype(c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
+#endif
+
 ASSERT_SAME_TYPE(size_t, decltype(mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(size_t, decltype(c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 
Index: libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
===================================================================
--- libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
+++ libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp
@@ -20,11 +20,14 @@
 
 #include "test_macros.h"
 
-// TODO: Implement mbrtoc8 and c8rtomb, and add tests for those
-
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
+#if defined(_LIBCPP_HAS_C8RTOMB_MBRTOC8)
+ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(size_t, decltype(std::c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
+#endif
+
 ASSERT_SAME_TYPE(size_t, decltype(std::mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(size_t, decltype(std::c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));