[libc++] Remove deprecated char_traits base template (#72694)

This patch has quite a bit of history. First, it must be noted that the
Standard only specifies specializations of char_traits for char,
char8_t, char16_t, char32_t and wchar_t. However, before this patch, we
would provide a base template that accepted anything, and as a result
code like `std::basic_string<long long>` would compile but nobody knows
what it really does. It basically compiles by accident.

We marked the base template as deprecated in LLVM 15 or 16 and were
planning on removing it in LLVM 17, which we did in e30a148.
However, it turned out that the deprecation warning had never been
visible in user code since Clang did not surface that warning from
system headers. As a result, this caught people by surprise and we
decided to reintroduce the base template in LLVM 17 in cce062d.

Since then, #70353 changed Clang so that such deprecation warnings would
be visible from user code. Hence, this patch closes the loop and removes
the deprecated specializations.

libcxx/docs/ReleaseNotes/19.rst

@@ -56,7 +56,7 @@ Deprecations and Removals
   the LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation
   <using-hardening-modes>` for more details.
 
-- TODO: The base template for ``std::char_traits`` has been removed in LLVM 19. If you are using ``std::char_traits`` with
+- The base template for ``std::char_traits`` has been removed in LLVM 19. If you are using ``std::char_traits`` with
   types other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``, ``char32_t`` or a custom character type for which you
   specialized ``std::char_traits``, your code will stop working. The Standard does not mandate that a base template is
   provided, and such a base template is bound to be incorrect for some types, which could currently cause unexpected behavior

libcxx/include/__string/char_traits.h

@@ -72,106 +72,6 @@ exposition-only to document what members a char_traits specialization should pro
 };
 */
 
-//
-// Temporary extension to provide a base template for std::char_traits.
-// TODO(LLVM-19): Remove this class.
-//
-#if !defined(_LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION)
-template <class _CharT>
-struct _LIBCPP_DEPRECATED_(
-    "char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided "
-    "for a temporary period. It will be removed in LLVM 19, so please migrate off of it.") char_traits {
-  using char_type  = _CharT;
-  using int_type   = int;
-  using off_type   = streamoff;
-  using pos_type   = streampos;
-  using state_type = mbstate_t;
-
-  static inline void _LIBCPP_CONSTEXPR_SINCE_CXX17 _LIBCPP_HIDE_FROM_ABI
-  assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {
-    __c1 = __c2;
-  }
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT {
-    return __c1 == __c2;
-  }
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT {
-    return __c1 < __c2;
-  }
-
-  static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 int
-  compare(const char_type* __s1, const char_type* __s2, size_t __n) {
-    for (; __n; --__n, ++__s1, ++__s2) {
-      if (lt(*__s1, *__s2))
-        return -1;
-      if (lt(*__s2, *__s1))
-        return 1;
-    }
-    return 0;
-  }
-  _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 size_t length(const char_type* __s) {
-    size_t __len = 0;
-    for (; !eq(*__s, char_type(0)); ++__s)
-      ++__len;
-    return __len;
-  }
-  _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX17 const char_type*
-  find(const char_type* __s, size_t __n, const char_type& __a) {
-    for (; __n; --__n) {
-      if (eq(*__s, __a))
-        return __s;
-      ++__s;
-    }
-    return nullptr;
-  }
-  static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 char_type*
-  move(char_type* __s1, const char_type* __s2, size_t __n) {
-    if (__n == 0)
-      return __s1;
-    char_type* __r = __s1;
-    if (__s1 < __s2) {
-      for (; __n; --__n, ++__s1, ++__s2)
-        assign(*__s1, *__s2);
-    } else if (__s2 < __s1) {
-      __s1 += __n;
-      __s2 += __n;
-      for (; __n; --__n)
-        assign(*--__s1, *--__s2);
-    }
-    return __r;
-  }
-  _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX20 char_type*
-  copy(char_type* __s1, const char_type* __s2, size_t __n) {
-    _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
-                                          "char_traits::copy: source and destination ranges overlap");
-    char_type* __r = __s1;
-    for (; __n; --__n, ++__s1, ++__s2)
-      assign(*__s1, *__s2);
-    return __r;
-  }
-  _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX20 char_type*
-  assign(char_type* __s, size_t __n, char_type __a) {
-    char_type* __r = __s;
-    for (; __n; --__n, ++__s)
-      assign(*__s, __a);
-    return __r;
-  }
-
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type not_eof(int_type __c) _NOEXCEPT {
-    return eq_int_type(__c, eof()) ? ~eof() : __c;
-  }
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT {
-    return char_type(__c);
-  }
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT {
-    return int_type(__c);
-  }
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT {
-    return __c1 == __c2;
-  }
-  static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT { return int_type(EOF); }
-};
-#endif // !defined(_LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION)
-
 // char_traits<char>
 
 template <>