diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -50,7 +50,7 @@
 #  elif __cplusplus <= 201703L
 #    define _LIBCPP_STD_VER 17
 #  else
-#    define _LIBCPP_STD_VER 18  // current year, or date of c++2a ratification
+#    define _LIBCPP_STD_VER 20  // current year, or date of c++2a ratification
 #  endif
 #endif  // _LIBCPP_STD_VER
 
@@ -997,6 +997,12 @@
 #  define _LIBCPP_DEPRECATED_IN_CXX17
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+#  define _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_DEPRECATED
+#else
+#  define _LIBCPP_DEPRECATED_IN_CXX20
+#endif
+
 // Macros to enter and leave a state where deprecation warnings are suppressed.
 #if !defined(_LIBCPP_SUPPRESS_DEPRECATED_PUSH) && \
     (defined(_LIBCPP_COMPILER_CLANG) || defined(_LIBCPP_COMPILER_GCC))
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -106,6 +106,11 @@
 };
 #endif
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+    typedef char8_t __char8_t;
+#else
+    typedef char    __char8_t;
+#endif
 
 class _LIBCPP_TYPE_VIS locale;
 
@@ -1072,10 +1077,11 @@
     virtual int do_max_length() const  _NOEXCEPT;
 };
 
-// template <> class codecvt<char16_t, char, mbstate_t>
+// template <> class codecvt<char16_t, char, mbstate_t> // deprecated in C++20
 
+// TODO: Should _LIBCPP_DEPRECATED_IN_CXX20 be present only if char8_t is supported??
 template <>
-class _LIBCPP_TYPE_VIS codecvt<char16_t, char, mbstate_t>
+class _LIBCPP_TYPE_VIS _LIBCPP_DEPRECATED_IN_CXX20 codecvt<char16_t, char, mbstate_t>
     : public locale::facet,
       public codecvt_base
 {
@@ -1158,10 +1164,101 @@
     virtual int do_max_length() const  _NOEXCEPT;
 };
 
-// template <> class codecvt<char32_t, char, mbstate_t>
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+
+// template <> class codecvt<char16_t, char8_t, mbstate_t> // C++20
+
+template <>
+class _LIBCPP_TYPE_VIS codecvt<char16_t, char8_t, mbstate_t>
+    : public locale::facet,
+      public codecvt_base
+{
+public:
+    typedef char16_t  intern_type;
+    typedef char8_t   extern_type;
+    typedef mbstate_t state_type;
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit codecvt(size_t __refs = 0)
+        : locale::facet(__refs) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    result out(state_type& __st,
+               const intern_type* __frm, const intern_type* __frm_end, const intern_type*& __frm_nxt,
+               extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const
+    {
+        return do_out(__st, __frm, __frm_end, __frm_nxt, __to, __to_end, __to_nxt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    result unshift(state_type& __st,
+                   extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const
+    {
+        return do_unshift(__st, __to, __to_end, __to_nxt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    result in(state_type& __st,
+              const extern_type* __frm, const extern_type* __frm_end, const extern_type*& __frm_nxt,
+              intern_type* __to, intern_type* __to_end, intern_type*& __to_nxt) const
+    {
+        return do_in(__st, __frm, __frm_end, __frm_nxt, __to, __to_end, __to_nxt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    int encoding() const  _NOEXCEPT
+    {
+        return do_encoding();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    bool always_noconv() const  _NOEXCEPT
+    {
+        return do_always_noconv();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    int length(state_type& __st, const extern_type* __frm, const extern_type* __end, size_t __mx) const
+    {
+        return do_length(__st, __frm, __end, __mx);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    int max_length() const  _NOEXCEPT
+    {
+        return do_max_length();
+    }
+
+    static locale::id id;
+
+protected:
+    _LIBCPP_INLINE_VISIBILITY
+    explicit codecvt(const char*, size_t __refs = 0)
+        : locale::facet(__refs) {}
+
+    ~codecvt();
+
+    virtual result do_out(state_type& __st,
+                          const intern_type* __frm, const intern_type* __frm_end, const intern_type*& __frm_nxt,
+                          extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const;
+    virtual result do_in(state_type& __st,
+                         const extern_type* __frm, const extern_type* __frm_end, const extern_type*& __frm_nxt,
+                         intern_type* __to, intern_type* __to_end, intern_type*& __to_nxt) const;
+    virtual result do_unshift(state_type& __st,
+                              extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const;
+    virtual int do_encoding() const  _NOEXCEPT;
+    virtual bool do_always_noconv() const  _NOEXCEPT;
+    virtual int do_length(state_type&, const extern_type* __frm, const extern_type* __end, size_t __mx) const;
+    virtual int do_max_length() const  _NOEXCEPT;
+};
+
+#endif
+
+// template <> class codecvt<char32_t, char, mbstate_t> // deprecated in C++20
 
+// TODO: Should _LIBCPP_DEPRECATED_IN_CXX20 be present only if char8_t is supported??
 template <>
-class _LIBCPP_TYPE_VIS codecvt<char32_t, char, mbstate_t>
+class _LIBCPP_TYPE_VIS _LIBCPP_DEPRECATED_IN_CXX20 codecvt<char32_t, char, mbstate_t>
     : public locale::facet,
       public codecvt_base
 {
@@ -1244,6 +1341,96 @@
     virtual int do_max_length() const  _NOEXCEPT;
 };
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+
+// template <> class codecvt<char32_t, char8_t, mbstate_t> // C++20
+
+template <>
+class _LIBCPP_TYPE_VIS codecvt<char32_t, char8_t, mbstate_t>
+    : public locale::facet,
+      public codecvt_base
+{
+public:
+    typedef char32_t  intern_type;
+    typedef char8_t   extern_type;
+    typedef mbstate_t state_type;
+
+    _LIBCPP_INLINE_VISIBILITY
+    explicit codecvt(size_t __refs = 0)
+        : locale::facet(__refs) {}
+
+    _LIBCPP_INLINE_VISIBILITY
+    result out(state_type& __st,
+               const intern_type* __frm, const intern_type* __frm_end, const intern_type*& __frm_nxt,
+               extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const
+    {
+        return do_out(__st, __frm, __frm_end, __frm_nxt, __to, __to_end, __to_nxt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    result unshift(state_type& __st,
+                   extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const
+    {
+        return do_unshift(__st, __to, __to_end, __to_nxt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    result in(state_type& __st,
+              const extern_type* __frm, const extern_type* __frm_end, const extern_type*& __frm_nxt,
+              intern_type* __to, intern_type* __to_end, intern_type*& __to_nxt) const
+    {
+        return do_in(__st, __frm, __frm_end, __frm_nxt, __to, __to_end, __to_nxt);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    int encoding() const  _NOEXCEPT
+    {
+        return do_encoding();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    bool always_noconv() const  _NOEXCEPT
+    {
+        return do_always_noconv();
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    int length(state_type& __st, const extern_type* __frm, const extern_type* __end, size_t __mx) const
+    {
+        return do_length(__st, __frm, __end, __mx);
+    }
+
+    _LIBCPP_INLINE_VISIBILITY
+    int max_length() const  _NOEXCEPT
+    {
+        return do_max_length();
+    }
+
+    static locale::id id;
+
+protected:
+    _LIBCPP_INLINE_VISIBILITY
+    explicit codecvt(const char*, size_t __refs = 0)
+        : locale::facet(__refs) {}
+
+    ~codecvt();
+
+    virtual result do_out(state_type& __st,
+                          const intern_type* __frm, const intern_type* __frm_end, const intern_type*& __frm_nxt,
+                          extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const;
+    virtual result do_in(state_type& __st,
+                         const extern_type* __frm, const extern_type* __frm_end, const extern_type*& __frm_nxt,
+                         intern_type* __to, intern_type* __to_end, intern_type*& __to_nxt) const;
+    virtual result do_unshift(state_type& __st,
+                              extern_type* __to, extern_type* __to_end, extern_type*& __to_nxt) const;
+    virtual int do_encoding() const  _NOEXCEPT;
+    virtual bool do_always_noconv() const  _NOEXCEPT;
+    virtual int do_length(state_type&, const extern_type* __frm, const extern_type* __end, size_t __mx) const;
+    virtual int do_max_length() const  _NOEXCEPT;
+};
+
+#endif
+
 // template <class _InternT, class _ExternT, class _StateT> class codecvt_byname
 
 template <class _InternT, class _ExternT, class _StateT>
@@ -1261,15 +1448,22 @@
     ~codecvt_byname();
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _InternT, class _ExternT, class _StateT>
 codecvt_byname<_InternT, _ExternT, _StateT>::~codecvt_byname()
 {
 }
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS codecvt_byname<char, char, mbstate_t>)
 _LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS codecvt_byname<wchar_t, char, mbstate_t>)
-_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS codecvt_byname<char16_t, char, mbstate_t>)
-_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS codecvt_byname<char32_t, char, mbstate_t>)
+// TODO: Should _LIBCPP_DEPRECATED_IN_CXX20 be present only if char8_t is supported??
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DEPRECATED_IN_CXX20 codecvt_byname<char16_t, char, mbstate_t>) // deprecated in C++20
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DEPRECATED_IN_CXX20 codecvt_byname<char32_t, char, mbstate_t>) // deprecated in C++20
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS codecvt_byname<char16_t, char8_t, mbstate_t>) // C++20
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS codecvt_byname<char32_t, char8_t, mbstate_t>) // C++20
+#endif
 
 template <size_t _Np>
 struct __narrow_to_utf8
@@ -1293,12 +1487,14 @@
     }
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 struct _LIBCPP_TEMPLATE_VIS __narrow_to_utf8<16>
     : public codecvt<char16_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __narrow_to_utf8() : codecvt<char16_t, char, mbstate_t>(1) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
     _LIBCPP_EXPORTED_FROM_ABI ~__narrow_to_utf8();
 
@@ -1327,12 +1523,14 @@
     }
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 struct _LIBCPP_TEMPLATE_VIS __narrow_to_utf8<32>
     : public codecvt<char32_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __narrow_to_utf8() : codecvt<char32_t, char, mbstate_t>(1) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
     _LIBCPP_EXPORTED_FROM_ABI ~__narrow_to_utf8();
 
@@ -1383,12 +1581,14 @@
     }
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 struct _LIBCPP_TEMPLATE_VIS __widen_from_utf8<16>
     : public codecvt<char16_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __widen_from_utf8() : codecvt<char16_t, char, mbstate_t>(1) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
     _LIBCPP_EXPORTED_FROM_ABI ~__widen_from_utf8();
 
@@ -1417,12 +1617,14 @@
     }
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 struct _LIBCPP_TEMPLATE_VIS __widen_from_utf8<32>
     : public codecvt<char32_t, char, mbstate_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     __widen_from_utf8() : codecvt<char32_t, char, mbstate_t>(1) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
     _LIBCPP_EXPORTED_FROM_ABI ~__widen_from_utf8();
 
diff --git a/libcxx/include/codecvt b/libcxx/include/codecvt
--- a/libcxx/include/codecvt
+++ b/libcxx/include/codecvt
@@ -109,6 +109,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf8<char16_t>
     : public codecvt<char16_t, char, mbstate_t>
@@ -125,6 +126,8 @@
                             codecvt_mode _Mode)
         : codecvt<char16_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -144,6 +147,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf8<char32_t>
     : public codecvt<char32_t, char, mbstate_t>
@@ -160,6 +164,8 @@
                             codecvt_mode _Mode)
         : codecvt<char32_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -267,6 +273,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf16<char16_t, false>
     : public codecvt<char16_t, char, mbstate_t>
@@ -283,6 +290,8 @@
                             codecvt_mode _Mode)
         : codecvt<char16_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -302,6 +311,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf16<char16_t, true>
     : public codecvt<char16_t, char, mbstate_t>
@@ -318,6 +328,8 @@
                             codecvt_mode _Mode)
         : codecvt<char16_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -337,6 +349,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf16<char32_t, false>
     : public codecvt<char32_t, char, mbstate_t>
@@ -353,6 +366,8 @@
                             codecvt_mode _Mode)
         : codecvt<char32_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -372,6 +387,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf16<char32_t, true>
     : public codecvt<char32_t, char, mbstate_t>
@@ -388,6 +404,8 @@
                             codecvt_mode _Mode)
         : codecvt<char32_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -460,6 +478,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf8_utf16<char32_t>
     : public codecvt<char32_t, char, mbstate_t>
@@ -476,6 +495,8 @@
                             codecvt_mode _Mode)
         : codecvt<char32_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
@@ -495,6 +516,7 @@
     virtual int do_max_length() const _NOEXCEPT;
 };
 
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <>
 class _LIBCPP_TYPE_VIS __codecvt_utf8_utf16<char16_t>
     : public codecvt<char16_t, char, mbstate_t>
@@ -511,6 +533,8 @@
                             codecvt_mode _Mode)
         : codecvt<char16_t, char, mbstate_t>(__refs), _Maxcode_(_Maxcode),
           _Mode_(_Mode) {}
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 protected:
     virtual result
         do_out(state_type& __st,
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -204,6 +204,10 @@
     install(&make<codecvt<wchar_t, char, mbstate_t> >(1u));
     install(&make<codecvt<char16_t, char, mbstate_t> >(1u));
     install(&make<codecvt<char32_t, char, mbstate_t> >(1u));
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+    install(&make<codecvt<char16_t, char8_t, mbstate_t> >(1u));
+    install(&make<codecvt<char32_t, char8_t, mbstate_t> >(1u));
+#endif
     install(&make<numpunct<char> >(1u));
     install(&make<numpunct<wchar_t> >(1u));
     install(&make<num_get<char> >(1u));
@@ -247,6 +251,10 @@
         install(new codecvt_byname<wchar_t, char, mbstate_t>(name_));
         install(new codecvt_byname<char16_t, char, mbstate_t>(name_));
         install(new codecvt_byname<char32_t, char, mbstate_t>(name_));
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+        install(new codecvt_byname<char16_t, char8_t, mbstate_t>(name_));
+        install(new codecvt_byname<char32_t, char8_t, mbstate_t>(name_));
+#endif
         install(new numpunct_byname<char>(name_));
         install(new numpunct_byname<wchar_t>(name_));
         install(new moneypunct_byname<char, false>(name_));
@@ -317,6 +325,10 @@
             install(new codecvt_byname<wchar_t, char, mbstate_t>(name));
             install(new codecvt_byname<char16_t, char, mbstate_t>(name));
             install(new codecvt_byname<char32_t, char, mbstate_t>(name));
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+            install(new codecvt_byname<char16_t, char8_t, mbstate_t>(name));
+            install(new codecvt_byname<char32_t, char8_t, mbstate_t>(name));
+#endif
         }
         if (c & locale::monetary)
         {
@@ -387,6 +399,10 @@
             install_from<_VSTD::codecvt<char, char, mbstate_t> >(one);
             install_from<_VSTD::codecvt<char16_t, char, mbstate_t> >(one);
             install_from<_VSTD::codecvt<char32_t, char, mbstate_t> >(one);
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+            install_from<_VSTD::codecvt<char16_t, char8_t, mbstate_t> >(one);
+            install_from<_VSTD::codecvt<char32_t, char8_t, mbstate_t> >(one);
+#endif
             install_from<_VSTD::codecvt<wchar_t, char, mbstate_t> >(one);
         }
         if (c & locale::monetary)
@@ -3171,6 +3187,87 @@
     return 4;
 }
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+
+// template <> class codecvt<char16_t, char8_t, mbstate_t>
+
+locale::id codecvt<char16_t, char8_t, mbstate_t>::id;
+
+codecvt<char16_t, char8_t, mbstate_t>::~codecvt()
+{
+}
+
+codecvt<char16_t, char8_t, mbstate_t>::result
+codecvt<char16_t, char8_t, mbstate_t>::do_out(state_type&,
+    const intern_type* frm, const intern_type* frm_end, const intern_type*& frm_nxt,
+    extern_type* to, extern_type* to_end, extern_type*& to_nxt) const
+{
+    const uint16_t* _frm = reinterpret_cast<const uint16_t*>(frm);
+    const uint16_t* _frm_end = reinterpret_cast<const uint16_t*>(frm_end);
+    const uint16_t* _frm_nxt = _frm;
+    uint8_t* _to = reinterpret_cast<uint8_t*>(to);
+    uint8_t* _to_end = reinterpret_cast<uint8_t*>(to_end);
+    uint8_t* _to_nxt = _to;
+    result r = utf16_to_utf8(_frm, _frm_end, _frm_nxt, _to, _to_end, _to_nxt);
+    frm_nxt = frm + (_frm_nxt - _frm);
+    to_nxt = to + (_to_nxt - _to);
+    return r;
+}
+
+codecvt<char16_t, char8_t, mbstate_t>::result
+codecvt<char16_t, char8_t, mbstate_t>::do_in(state_type&,
+    const extern_type* frm, const extern_type* frm_end, const extern_type*& frm_nxt,
+    intern_type* to, intern_type* to_end, intern_type*& to_nxt) const
+{
+    const uint8_t* _frm = reinterpret_cast<const uint8_t*>(frm);
+    const uint8_t* _frm_end = reinterpret_cast<const uint8_t*>(frm_end);
+    const uint8_t* _frm_nxt = _frm;
+    uint16_t* _to = reinterpret_cast<uint16_t*>(to);
+    uint16_t* _to_end = reinterpret_cast<uint16_t*>(to_end);
+    uint16_t* _to_nxt = _to;
+    result r = utf8_to_utf16(_frm, _frm_end, _frm_nxt, _to, _to_end, _to_nxt);
+    frm_nxt = frm + (_frm_nxt - _frm);
+    to_nxt = to + (_to_nxt - _to);
+    return r;
+}
+
+codecvt<char16_t, char8_t, mbstate_t>::result
+codecvt<char16_t, char8_t, mbstate_t>::do_unshift(state_type&,
+    extern_type* to, extern_type*, extern_type*& to_nxt) const
+{
+    to_nxt = to;
+    return noconv;
+}
+
+int
+codecvt<char16_t, char8_t, mbstate_t>::do_encoding() const  _NOEXCEPT
+{
+    return 0;
+}
+
+bool
+codecvt<char16_t, char8_t, mbstate_t>::do_always_noconv() const  _NOEXCEPT
+{
+    return false;
+}
+
+int
+codecvt<char16_t, char8_t, mbstate_t>::do_length(state_type&,
+    const extern_type* frm, const extern_type* frm_end, size_t mx) const
+{
+    const uint8_t* _frm = reinterpret_cast<const uint8_t*>(frm);
+    const uint8_t* _frm_end = reinterpret_cast<const uint8_t*>(frm_end);
+    return utf8_to_utf16_length(_frm, _frm_end, mx);
+}
+
+int
+codecvt<char16_t, char8_t, mbstate_t>::do_max_length() const  _NOEXCEPT
+{
+    return 4;
+}
+
+#endif
+
 // template <> class codecvt<char32_t, char, mbstate_t>
 
 locale::id codecvt<char32_t, char, mbstate_t>::id;
@@ -3248,6 +3345,87 @@
     return 4;
 }
 
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+
+// template <> class codecvt<char32_t, char8_t, mbstate_t>
+
+locale::id codecvt<char32_t, char8_t, mbstate_t>::id;
+
+codecvt<char32_t, char8_t, mbstate_t>::~codecvt()
+{
+}
+
+codecvt<char32_t, char8_t, mbstate_t>::result
+codecvt<char32_t, char8_t, mbstate_t>::do_out(state_type&,
+    const intern_type* frm, const intern_type* frm_end, const intern_type*& frm_nxt,
+    extern_type* to, extern_type* to_end, extern_type*& to_nxt) const
+{
+    const uint32_t* _frm = reinterpret_cast<const uint32_t*>(frm);
+    const uint32_t* _frm_end = reinterpret_cast<const uint32_t*>(frm_end);
+    const uint32_t* _frm_nxt = _frm;
+    uint8_t* _to = reinterpret_cast<uint8_t*>(to);
+    uint8_t* _to_end = reinterpret_cast<uint8_t*>(to_end);
+    uint8_t* _to_nxt = _to;
+    result r = ucs4_to_utf8(_frm, _frm_end, _frm_nxt, _to, _to_end, _to_nxt);
+    frm_nxt = frm + (_frm_nxt - _frm);
+    to_nxt = to + (_to_nxt - _to);
+    return r;
+}
+
+codecvt<char32_t, char8_t, mbstate_t>::result
+codecvt<char32_t, char8_t, mbstate_t>::do_in(state_type&,
+    const extern_type* frm, const extern_type* frm_end, const extern_type*& frm_nxt,
+    intern_type* to, intern_type* to_end, intern_type*& to_nxt) const
+{
+    const uint8_t* _frm = reinterpret_cast<const uint8_t*>(frm);
+    const uint8_t* _frm_end = reinterpret_cast<const uint8_t*>(frm_end);
+    const uint8_t* _frm_nxt = _frm;
+    uint32_t* _to = reinterpret_cast<uint32_t*>(to);
+    uint32_t* _to_end = reinterpret_cast<uint32_t*>(to_end);
+    uint32_t* _to_nxt = _to;
+    result r = utf8_to_ucs4(_frm, _frm_end, _frm_nxt, _to, _to_end, _to_nxt);
+    frm_nxt = frm + (_frm_nxt - _frm);
+    to_nxt = to + (_to_nxt - _to);
+    return r;
+}
+
+codecvt<char32_t, char8_t, mbstate_t>::result
+codecvt<char32_t, char8_t, mbstate_t>::do_unshift(state_type&,
+    extern_type* to, extern_type*, extern_type*& to_nxt) const
+{
+    to_nxt = to;
+    return noconv;
+}
+
+int
+codecvt<char32_t, char8_t, mbstate_t>::do_encoding() const  _NOEXCEPT
+{
+    return 0;
+}
+
+bool
+codecvt<char32_t, char8_t, mbstate_t>::do_always_noconv() const  _NOEXCEPT
+{
+    return false;
+}
+
+int
+codecvt<char32_t, char8_t, mbstate_t>::do_length(state_type&,
+    const extern_type* frm, const extern_type* frm_end, size_t mx) const
+{
+    const uint8_t* _frm = reinterpret_cast<const uint8_t*>(frm);
+    const uint8_t* _frm_end = reinterpret_cast<const uint8_t*>(frm_end);
+    return utf8_to_ucs4_length(_frm, _frm_end, mx);
+}
+
+int
+codecvt<char32_t, char8_t, mbstate_t>::do_max_length() const  _NOEXCEPT
+{
+    return 4;
+}
+
+#endif
+
 // __codecvt_utf8<wchar_t>
 
 __codecvt_utf8<wchar_t>::result
@@ -6148,7 +6326,11 @@
 
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS codecvt_byname<char, char, mbstate_t>;
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS codecvt_byname<wchar_t, char, mbstate_t>;
-template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS codecvt_byname<char16_t, char, mbstate_t>;
-template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS codecvt_byname<char32_t, char, mbstate_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DEPRECATED_IN_CXX20 codecvt_byname<char16_t, char, mbstate_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DEPRECATED_IN_CXX20 codecvt_byname<char32_t, char, mbstate_t>;
+#ifndef _LIBCPP_NO_HAS_CHAR8_T
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS codecvt_byname<char16_t, char8_t, mbstate_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS codecvt_byname<char32_t, char8_t, mbstate_t>;
+#endif
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/codecvt_byname_char16_t_char.depr_in_cxx20.verify.cpp
copy from libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
copy to libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/codecvt_byname_char16_t_char.depr_in_cxx20.verify.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/codecvt_byname_char16_t_char.depr_in_cxx20.verify.cpp
@@ -8,22 +8,20 @@
 
 // <locale>
 
-// template <> class codecvt<char16_t, char, mbstate_t>
+// codecvt_byname<char16_t, char, mbstate_t>
+//  deprecated in C++20
 
-// bool always_noconv() const throw();
+// UNSUPPORTED: c++03, c++11, c++14, c++17
 
 #include <locale>
-#include <cassert>
 
+#include "../with_public_dtor.hpp"
 #include "test_macros.h"
 
-typedef std::codecvt<char16_t, char, std::mbstate_t> F;
-
-int main(int, char**)
+int main(int, char **)
 {
-    std::locale l = std::locale::classic();
-    const F& f = std::use_facet<F>(l);
-    assert(!f.always_noconv());
+    with_public_dtor<std::codecvt_byname<char16_t, char, std::mbstate_t>> cvt("", 0); // expected-warning {{'codecvt_byname<char16_t, char, __mbstate_t>' is deprecated}}
+    (void)cvt;
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/codecvt_byname_char32_t_char.depr_in_cxx20.verify.cpp
copy from libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
copy to libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/codecvt_byname_char32_t_char.depr_in_cxx20.verify.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/codecvt_byname_char32_t_char.depr_in_cxx20.verify.cpp
@@ -8,22 +8,20 @@
 
 // <locale>
 
-// template <> class codecvt<char16_t, char, mbstate_t>
+// codecvt_byname<char32_t, char, mbstate_t>
+//  deprecated in C++20
 
-// bool always_noconv() const throw();
+// UNSUPPORTED: c++03, c++11, c++14, c++17
 
 #include <locale>
-#include <cassert>
 
+#include "../with_public_dtor.hpp"
 #include "test_macros.h"
 
-typedef std::codecvt<char16_t, char, std::mbstate_t> F;
-
-int main(int, char**)
+int main(int, char **)
 {
-    std::locale l = std::locale::classic();
-    const F& f = std::use_facet<F>(l);
-    assert(!f.always_noconv());
+    with_public_dtor<std::codecvt_byname<char32_t, char, std::mbstate_t>> cvt("", 0); // expected-warning {{'codecvt_byname<char32_t, char, __mbstate_t>' is deprecated}}
+    (void)cvt;
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
@@ -13,6 +13,9 @@
 // explicit codecvt_byname(const char* nm, size_t refs = 0);
 // explicit codecvt_byname(const string& nm, size_t refs = 0);
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t_char8_t.pass.cpp
copy from libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
copy to libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t_char8_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t_char8_t.pass.cpp
@@ -8,17 +8,19 @@
 
 // <locale>
 
-// template <> class codecvt_byname<char16_t, char, mbstate_t>
+// template <> class codecvt_byname<char16_t, char8_t, mbstate_t>
 
 // explicit codecvt_byname(const char* nm, size_t refs = 0);
 // explicit codecvt_byname(const string& nm, size_t refs = 0);
 
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
 #include <locale>
 #include <cassert>
 
 #include "test_macros.h"
 
-typedef std::codecvt_byname<char16_t, char, std::mbstate_t> F;
+typedef std::codecvt_byname<char16_t, char8_t, std::mbstate_t> F;
 
 class my_facet
     : public F
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t.pass.cpp
@@ -13,6 +13,9 @@
 // explicit codecvt_byname(const char* nm, size_t refs = 0);
 // explicit codecvt_byname(const string& nm, size_t refs = 0);
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t_char8_t.pass.cpp
copy from libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
copy to libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t_char8_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char16_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt.byname/ctor_char32_t_char8_t.pass.cpp
@@ -8,17 +8,19 @@
 
 // <locale>
 
-// template <> class codecvt_byname<char16_t, char, mbstate_t>
+// template <> class codecvt_byname<char32_t, char8_t, mbstate_t>
 
 // explicit codecvt_byname(const char* nm, size_t refs = 0);
 // explicit codecvt_byname(const string& nm, size_t refs = 0);
 
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
 #include <locale>
 #include <cassert>
 
 #include "test_macros.h"
 
-typedef std::codecvt_byname<char16_t, char, std::mbstate_t> F;
+typedef std::codecvt_byname<char32_t, char8_t, std::mbstate_t> F;
 
 class my_facet
     : public F
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/codecvt_char16_t_char.depr_in_cxx20.verify.cpp
copy from libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
copy to libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/codecvt_char16_t_char.depr_in_cxx20.verify.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/codecvt_char16_t_char.depr_in_cxx20.verify.cpp
@@ -8,22 +8,20 @@
 
 // <locale>
 
-// template <> class codecvt<char16_t, char, mbstate_t>
+// codecvt<char16_t, char, mbstate_t>
+//  deprecated in C++20
 
-// bool always_noconv() const throw();
+// UNSUPPORTED: c++03, c++11, c++14, c++17
 
 #include <locale>
-#include <cassert>
 
+#include "../with_public_dtor.hpp"
 #include "test_macros.h"
 
-typedef std::codecvt<char16_t, char, std::mbstate_t> F;
-
-int main(int, char**)
+int main(int, char **)
 {
-    std::locale l = std::locale::classic();
-    const F& f = std::use_facet<F>(l);
-    assert(!f.always_noconv());
+    with_public_dtor<std::codecvt<char16_t, char, std::mbstate_t>> cvt("", 0); // expected-warning {{'codecvt<char16_t, char, __mbstate_t>' is deprecated}}
+    (void)cvt;
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/codecvt_char32_t_char.depr_in_cxx20.verify.cpp
copy from libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
copy to libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/codecvt_char32_t_char.depr_in_cxx20.verify.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/codecvt_char32_t_char.depr_in_cxx20.verify.cpp
@@ -8,22 +8,20 @@
 
 // <locale>
 
-// template <> class codecvt<char16_t, char, mbstate_t>
+// codecvt<char32_t, char, mbstate_t>
+//  deprecated in C++20
 
-// bool always_noconv() const throw();
+// UNSUPPORTED: c++03, c++11, c++14, c++17
 
 #include <locale>
-#include <cassert>
 
+#include "../with_public_dtor.hpp"
 #include "test_macros.h"
 
-typedef std::codecvt<char16_t, char, std::mbstate_t> F;
-
-int main(int, char**)
+int main(int, char **)
 {
-    std::locale l = std::locale::classic();
-    const F& f = std::use_facet<F>(l);
-    assert(!f.always_noconv());
+    with_public_dtor<std::codecvt<char32_t, char, std::mbstate_t>> cvt("", 0); // expected-warning {{'codecvt<char32_t, char, __mbstate_t>' is deprecated}}
+    (void)cvt;
 
-  return 0;
+    return 0;
 }
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t.pass.cpp
@@ -12,6 +12,9 @@
 
 // explicit codecvt(size_t refs = 0);
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t_char8_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t_char8_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t_char8_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char16_t_char8_t.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t.pass.cpp
@@ -12,6 +12,9 @@
 
 // explicit codecvt(size_t refs = 0);
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t_char8_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t_char8_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t_char8_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/ctor_char32_t_char8_t.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_always_noconv.pass.cpp
@@ -12,6 +12,9 @@
 
 // bool always_noconv() const throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_always_noconv.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_always_noconv.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_encoding.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_encoding.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_encoding.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_encoding.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_in.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_in.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_in.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_in.pass.cpp
@@ -16,9 +16,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_length.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_max_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_max_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_max_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_max_length.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_out.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_out.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_out.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_out.pass.cpp
@@ -16,9 +16,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_unshift.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_unshift.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_unshift.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_char8_t_unshift.pass.cpp
@@ -15,9 +15,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_encoding.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_encoding.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_encoding.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_encoding.pass.cpp
@@ -12,6 +12,9 @@
 
 // int encoding() const throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_in.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_in.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_in.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_in.pass.cpp
@@ -14,6 +14,9 @@
 //           const externT* from, const externT* from_end, const externT*& from_next,
 //           internT* to, internT* to_end, internT*& to_next) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <string>
 #include <vector>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_length.pass.cpp
@@ -12,6 +12,9 @@
 
 // int length(stateT& state, const externT* from, const externT* from_end, size_t max) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_max_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_max_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_max_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_max_length.pass.cpp
@@ -12,6 +12,9 @@
 
 // int max_length() const throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_out.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_out.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_out.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_out.pass.cpp
@@ -14,6 +14,9 @@
 //            const internT* from, const internT* from_end, const internT*& from_next,
 //            externT* to, externT* to_end, externT*& to_next) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <string>
 #include <vector>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_unshift.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_unshift.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_unshift.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char16_t_unshift.pass.cpp
@@ -13,6 +13,9 @@
 // result unshift(stateT& state,
 //                externT* to, externT* to_end, externT*& to_next) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <string>
 #include <vector>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_always_noconv.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_always_noconv.pass.cpp
@@ -12,6 +12,9 @@
 
 // bool always_noconv() const throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_always_noconv.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_always_noconv.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_always_noconv.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_always_noconv.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_encoding.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_encoding.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_encoding.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_encoding.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_in.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_in.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_in.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_in.pass.cpp
@@ -16,9 +16,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_length.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_max_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_max_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_max_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_max_length.pass.cpp
@@ -14,9 +14,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_out.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_out.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_out.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_out.pass.cpp
@@ -16,9 +16,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_unshift.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_unshift.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_unshift.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_char8_t_unshift.pass.cpp
@@ -15,9 +15,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_encoding.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_encoding.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_encoding.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_encoding.pass.cpp
@@ -12,6 +12,9 @@
 
 // int encoding() const throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_in.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_in.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_in.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_in.pass.cpp
@@ -14,6 +14,9 @@
 //           const externT* from, const externT* from_end, const externT*& from_next,
 //           internT* to, internT* to_end, internT*& to_next) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <string>
 #include <vector>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_length.pass.cpp
@@ -12,6 +12,9 @@
 
 // int length(stateT& state, const externT* from, const externT* from_end, size_t max) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_max_length.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_max_length.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_max_length.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_max_length.pass.cpp
@@ -12,6 +12,9 @@
 
 // int max_length() const throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_out.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_out.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_out.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_out.pass.cpp
@@ -14,6 +14,9 @@
 //            const internT* from, const internT* from_end, const internT*& from_next,
 //            externT* to, externT* to_end, externT*& to_next) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <string>
 #include <vector>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_unshift.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_unshift.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_unshift.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/char32_t_unshift.pass.cpp
@@ -13,6 +13,9 @@
 // result unshift(stateT& state,
 //                externT* to, externT* to_end, externT*& to_next) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <string>
 #include <vector>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/utf_sanity_check.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/utf_sanity_check.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/utf_sanity_check.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/locale.codecvt.members/utf_sanity_check.pass.cpp
@@ -16,6 +16,9 @@
 
 // sanity check
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <codecvt>
 #include <cassert>
@@ -47,8 +50,7 @@
   char* c16cp;
   F32_8::extern_type c8[4];
 
-// C++20 codecvt specializations for char8_t are not yet implemented
-#if TEST_STD_VER > 17 && !defined(_LIBCPP_VERSION)
+#if TEST_STD_VER > 17
   typedef std::codecvt<char32_t, char8_t, std::mbstate_t> F32_8T;
   typedef std::codecvt<char16_t, char8_t, std::mbstate_t> F16_8T;
   const F32_8T& f32_8t = std::use_facet<F32_8T>(std::locale::classic());
@@ -69,8 +71,7 @@
              F32_8::error);
 #endif
 
-// C++20 codecvt specializations for char8_t are not yet implemented
-#if TEST_STD_VER > 17 && !defined(_LIBCPP_VERSION)
+#if TEST_STD_VER > 17
       assert(f32_8t.out(mbs32_8t, &c32x, &c32x + 1, c_c32p, c8t, c8t + 4,
                         c8tp) == F32_8T::error);
 #endif
@@ -153,8 +154,7 @@
       assert(c32p - &c32 == 1);
       assert(c32 == c32x);
 
-// C++20 codecvt specializations for char8_t are not yet implemented
-#if TEST_STD_VER > 17 && !defined(_LIBCPP_VERSION)
+#if TEST_STD_VER > 17
       assert(f32_8t.out(mbs32_8t, &c32x, &c32x + 1, c_c32p, c8t, c8t + 4,
                         c8tp) == F32_8T::ok);
       assert(c_c32p - &c32x == 1);
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t.pass.cpp
@@ -20,6 +20,9 @@
 //     ...
 // };
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <type_traits>
 #include <cassert>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t_char8_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t_char8_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t_char8_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char16_t_char8_t.pass.cpp
@@ -22,9 +22,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 #include <type_traits>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t.pass.cpp
@@ -20,6 +20,9 @@
 //     ...
 // };
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <type_traits>
 #include <cassert>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t_char8_t.pass.cpp b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t_char8_t.pass.cpp
--- a/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t_char8_t.pass.cpp
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/locale.codecvt/types_char32_t_char8_t.pass.cpp
@@ -22,9 +22,6 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// C++20 codecvt specializations for char8_t are not yet implemented:
-// UNSUPPORTED: libc++
-
 #include <cassert>
 #include <locale>
 #include <type_traits>
diff --git a/libcxx/test/std/localization/locale.categories/category.ctype/with_public_dtor.hpp b/libcxx/test/std/localization/locale.categories/category.ctype/with_public_dtor.hpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/localization/locale.categories/category.ctype/with_public_dtor.hpp
@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <utility>
+
+template <typename T>
+struct with_public_dtor : T
+{
+    template <typename... Args>
+    explicit with_public_dtor(Args &&... args)
+        : T(std::forward<Args>(args)...)
+    {
+    }
+};
diff --git a/libcxx/test/std/localization/locales/locale/locale.cons/default.pass.cpp b/libcxx/test/std/localization/locales/locale/locale.cons/default.pass.cpp
--- a/libcxx/test/std/localization/locales/locale/locale.cons/default.pass.cpp
+++ b/libcxx/test/std/localization/locales/locale/locale.cons/default.pass.cpp
@@ -12,6 +12,9 @@
 
 // locale() throw();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
@@ -29,6 +32,10 @@
     assert((std::has_facet<std::codecvt<char, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char16_t, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char32_t, char, std::mbstate_t> >(loc)));
+#if TEST_STD_VER > 17
+    assert((std::has_facet<std::codecvt<char16_t, char8_t, std::mbstate_t> >(loc)));
+    assert((std::has_facet<std::codecvt<char32_t, char8_t, std::mbstate_t> >(loc)));
+#endif
     assert((std::has_facet<std::codecvt<wchar_t, char, std::mbstate_t> >(loc)));
 
     assert((std::has_facet<std::moneypunct<char> >(loc)));
diff --git a/libcxx/test/std/localization/locales/locale/locale.members/combine.pass.cpp b/libcxx/test/std/localization/locales/locale/locale.members/combine.pass.cpp
--- a/libcxx/test/std/localization/locales/locale/locale.members/combine.pass.cpp
+++ b/libcxx/test/std/localization/locales/locale/locale.members/combine.pass.cpp
@@ -10,6 +10,9 @@
 
 // template <class Facet> locale combine(const locale& other) const;
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <stdexcept>
 #include <cassert>
@@ -28,6 +31,10 @@
     assert((std::has_facet<std::codecvt<char, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char16_t, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char32_t, char, std::mbstate_t> >(loc)));
+#if TEST_STD_VER > 17
+    assert((std::has_facet<std::codecvt<char16_t, char8_t, std::mbstate_t> >(loc)));
+    assert((std::has_facet<std::codecvt<char32_t, char8_t, std::mbstate_t> >(loc)));
+#endif
     assert((std::has_facet<std::codecvt<wchar_t, char, std::mbstate_t> >(loc)));
 
     assert((std::has_facet<std::moneypunct<char> >(loc)));
diff --git a/libcxx/test/std/localization/locales/locale/locale.statics/classic.pass.cpp b/libcxx/test/std/localization/locales/locale/locale.statics/classic.pass.cpp
--- a/libcxx/test/std/localization/locales/locale/locale.statics/classic.pass.cpp
+++ b/libcxx/test/std/localization/locales/locale/locale.statics/classic.pass.cpp
@@ -10,6 +10,9 @@
 
 // static const locale& classic();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
@@ -25,6 +28,10 @@
     assert((std::has_facet<std::codecvt<char, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char16_t, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char32_t, char, std::mbstate_t> >(loc)));
+#if TEST_STD_VER > 17
+    assert((std::has_facet<std::codecvt<char16_t, char8_t, std::mbstate_t> >(loc)));
+    assert((std::has_facet<std::codecvt<char32_t, char8_t, std::mbstate_t> >(loc)));
+#endif
     assert((std::has_facet<std::codecvt<wchar_t, char, std::mbstate_t> >(loc)));
 
     assert((std::has_facet<std::moneypunct<char> >(loc)));
diff --git a/libcxx/test/std/localization/locales/locale/locale.statics/global.pass.cpp b/libcxx/test/std/localization/locales/locale/locale.statics/global.pass.cpp
--- a/libcxx/test/std/localization/locales/locale/locale.statics/global.pass.cpp
+++ b/libcxx/test/std/localization/locales/locale/locale.statics/global.pass.cpp
@@ -12,6 +12,9 @@
 
 // static const locale& classic();
 
+// This test runs in C++20, but we have deprecated codecvt<char(16|32), char, mbstate_t> in C++20.
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS 
+
 #include <locale>
 #include <cassert>
 
@@ -28,6 +31,10 @@
     assert((std::has_facet<std::codecvt<char, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char16_t, char, std::mbstate_t> >(loc)));
     assert((std::has_facet<std::codecvt<char32_t, char, std::mbstate_t> >(loc)));
+#if TEST_STD_VER > 17
+    assert((std::has_facet<std::codecvt<char16_t, char8_t, std::mbstate_t> >(loc)));
+    assert((std::has_facet<std::codecvt<char32_t, char8_t, std::mbstate_t> >(loc)));
+#endif
     assert((std::has_facet<std::codecvt<wchar_t, char, std::mbstate_t> >(loc)));
 
     assert((std::has_facet<std::moneypunct<char> >(loc)));