[libc++] Remove deprecated char_traits base template

[ldionne]

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.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

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.

TODO: This will be landed in the LLVM 19 time frame, not before.
TODO: This patch also needs to update the release notes for LLVM 19 once we have some.

---

Full diff: /~https://github.com/llvm/llvm-project/pull/72694.diff

3 Files Affected:

• (modified) libcxx/include/__string/char_traits.h (-102)
• (removed) libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.deprecated.verify.cpp (-21)
• (removed) libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.pass.cpp (-146)

diff --git a/libcxx/include/__string/char_traits.h b/libcxx/include/__string/char_traits.h
index 005df9a98f17a28..baf2d2346a59601 100644
--- a/libcxx/include/__string/char_traits.h
+++ b/libcxx/include/__string/char_traits.h
@@ -71,108 +71,6 @@ 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_INLINE_VISIBILITY 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_INLINE_VISIBILITY 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_INLINE_VISIBILITY
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) {
-        if (!__libcpp_is_constant_evaluated()) {
-            _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(
-                __s2 < __s1 || __s2 >= __s1 + __n, "char_traits::copy overlapped range");
-        }
-        char_type* __r = __s1;
-        for (; __n; --__n, ++__s1, ++__s2)
-            assign(*__s1, *__s2);
-        return __r;
-    }
-    _LIBCPP_INLINE_VISIBILITY
-    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 <>
diff --git a/libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.deprecated.verify.cpp b/libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.deprecated.verify.cpp
deleted file mode 100644
index ec6f34ef5462e65..000000000000000
--- a/libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.deprecated.verify.cpp
+++ /dev/null
@@ -1,21 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// <string>
-
-// template<> struct char_traits<T> for arbitrary T
-
-// Make sure we issue deprecation warnings.
-
-#include <string>
-
-void f() {
-    std::char_traits<unsigned char> t1; (void)t1; // expected-warning{{'char_traits<unsigned char>' is deprecated}}
-    std::char_traits<signed char> t2; (void)t2; // expected-warning{{'char_traits<signed char>' is deprecated}}
-    std::char_traits<unsigned long> t3; (void)t3; // expected-warning{{'char_traits<unsigned long>' is deprecated}}
-}
diff --git a/libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.pass.cpp b/libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.pass.cpp
deleted file mode 100644
index c2de29d22b2fe4e..000000000000000
--- a/libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.pass.cpp
+++ /dev/null
@@ -1,146 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// <string>
-
-// template<> struct char_traits<T> for arbitrary T
-
-// Non-standard but provided temporarily for users to migrate.
-
-// ADDITIONAL_COMPILE_FLAGS: -Wno-deprecated
-
-#include <string>
-#include <cassert>
-#include <type_traits>
-
-#include "test_macros.h"
-
-template <class Char>
-TEST_CONSTEXPR_CXX20 bool test() {
-    static_assert(std::is_same<typename std::char_traits<Char>::char_type, Char>::value, "");
-    static_assert(std::is_same<typename std::char_traits<Char>::int_type, int>::value, "");
-    static_assert(std::is_same<typename std::char_traits<Char>::off_type, std::streamoff>::value, "");
-    static_assert(std::is_same<typename std::char_traits<Char>::pos_type, std::streampos>::value, "");
-    static_assert(std::is_same<typename std::char_traits<Char>::state_type, std::mbstate_t>::value, "");
-
-    assert(std::char_traits<Char>::to_int_type(Char('a')) == Char('a'));
-    assert(std::char_traits<Char>::to_int_type(Char('A')) == Char('A'));
-    assert(std::char_traits<Char>::to_int_type(0) == 0);
-
-    assert(std::char_traits<Char>::to_char_type(Char('a')) == Char('a'));
-    assert(std::char_traits<Char>::to_char_type(Char('A')) == Char('A'));
-    assert(std::char_traits<Char>::to_char_type(0) == 0);
-
-    assert(std::char_traits<Char>::eof() == EOF);
-
-    assert(std::char_traits<Char>::not_eof(Char('a')) == Char('a'));
-    assert(std::char_traits<Char>::not_eof(Char('A')) == Char('A'));
-    assert(std::char_traits<Char>::not_eof(0) == 0);
-    assert(std::char_traits<Char>::not_eof(std::char_traits<Char>::eof()) !=
-           std::char_traits<Char>::eof());
-
-    assert(std::char_traits<Char>::lt(Char('\0'), Char('A')) == (Char('\0') < Char('A')));
-    assert(std::char_traits<Char>::lt(Char('A'), Char('\0')) == (Char('A') < Char('\0')));
-    assert(std::char_traits<Char>::lt(Char('a'), Char('a')) == (Char('a') < Char('a')));
-    assert(std::char_traits<Char>::lt(Char('A'), Char('a')) == (Char('A') < Char('a')));
-    assert(std::char_traits<Char>::lt(Char('a'), Char('A')) == (Char('a') < Char('A')));
-
-    assert( std::char_traits<Char>::eq(Char('a'), Char('a')));
-    assert(!std::char_traits<Char>::eq(Char('a'), Char('A')));
-
-    assert( std::char_traits<Char>::eq_int_type(Char('a'), Char('a')));
-    assert(!std::char_traits<Char>::eq_int_type(Char('a'), Char('A')));
-    assert(!std::char_traits<Char>::eq_int_type(std::char_traits<Char>::eof(), Char('A')));
-    assert( std::char_traits<Char>::eq_int_type(std::char_traits<Char>::eof(), std::char_traits<Char>::eof()));
-
-    {
-        Char s1[] = {1, 2, 3, 0};
-        Char s2[] = {0};
-        assert(std::char_traits<Char>::length(s1) == 3);
-        assert(std::char_traits<Char>::length(s2) == 0);
-    }
-
-    {
-        Char s1[] = {1, 2, 3};
-        assert(std::char_traits<Char>::find(s1, 3, Char(1)) == s1);
-        assert(std::char_traits<Char>::find(s1, 3, Char(2)) == s1+1);
-        assert(std::char_traits<Char>::find(s1, 3, Char(3)) == s1+2);
-        assert(std::char_traits<Char>::find(s1, 3, Char(4)) == 0);
-        assert(std::char_traits<Char>::find(s1, 3, Char(0)) == 0);
-        assert(std::char_traits<Char>::find(NULL, 0, Char(0)) == 0);
-    }
-
-    {
-        Char s1[] = {1, 2, 3};
-        Char s2[3] = {0};
-        assert(std::char_traits<Char>::copy(s2, s1, 3) == s2);
-        assert(s2[0] == Char(1));
-        assert(s2[1] == Char(2));
-        assert(s2[2] == Char(3));
-        assert(std::char_traits<Char>::copy(NULL, s1, 0) == NULL);
-        assert(std::char_traits<Char>::copy(s1, NULL, 0) == s1);
-    }
-
-    {
-        Char s1[] = {1, 2, 3};
-        assert(std::char_traits<Char>::move(s1, s1+1, 2) == s1);
-        assert(s1[0] == Char(2));
-        assert(s1[1] == Char(3));
-        assert(s1[2] == Char(3));
-        s1[2] = Char(0);
-        assert(std::char_traits<Char>::move(s1+1, s1, 2) == s1+1);
-        assert(s1[0] == Char(2));
-        assert(s1[1] == Char(2));
-        assert(s1[2] == Char(3));
-        assert(std::char_traits<Char>::move(NULL, s1, 0) == NULL);
-        assert(std::char_traits<Char>::move(s1, NULL, 0) == s1);
-    }
-
-    {
-        Char s1[] = {0};
-        assert(std::char_traits<Char>::compare(s1, s1, 0) == 0);
-        assert(std::char_traits<Char>::compare(NULL, NULL, 0) == 0);
-
-        Char s2[] = {1, 0};
-        Char s3[] = {2, 0};
-        assert(std::char_traits<Char>::compare(s2, s2, 1) == 0);
-        assert(std::char_traits<Char>::compare(s2, s3, 1) < 0);
-        assert(std::char_traits<Char>::compare(s3, s2, 1) > 0);
-    }
-
-    {
-        Char s2[3] = {0};
-        assert(std::char_traits<Char>::assign(s2, 3, Char(5)) == s2);
-        assert(s2[0] == Char(5));
-        assert(s2[1] == Char(5));
-        assert(s2[2] == Char(5));
-        assert(std::char_traits<Char>::assign(NULL, 0, Char(5)) == NULL);
-    }
-
-    {
-        Char c = Char('\0');
-        std::char_traits<Char>::assign(c, Char('a'));
-        assert(c == Char('a'));
-    }
-
-    return true;
-}
-
-int main(int, char**) {
-    test<unsigned char>();
-    test<signed char>();
-    test<unsigned long>();
-
-#if TEST_STD_VER > 17
-    static_assert(test<unsigned char>());
-    static_assert(test<signed char>());
-    static_assert(test<unsigned long>());
-#endif
-
-  return 0;
-}

[ldionne]

@cor3ntin Thanks a bunch for fixing the underlying Clang issue and enabling this!

[EricWF]

Could you please give me a few days to test this internally?

[ldionne]

Could you please give me a few days to test this internally?

Sure, I mean this is going to land in the LLVM 19 time frame only anyways. But I would recommend doing the work now because otherwise this will become a problem for folks living at head when this lands.

CC @llvm/libcxx-vendors

[ldionne]

This should be ready for review now.

[ldionne]

CC @llvm/libcxx-vendors since this may break some (invalid) code.

[tambry]

Just for reference, an example of such breakage: jtv/libpqxx#751

[ldionne]

I'm going for it -- we've been trying to remove this for like 2 years and we went through a deprecation period (LLVM 18), so I think we should land this as early as possible in the release cycle.