[libc++][format] Addresses LWG3833.

[Mordante]

Summary

LWG3833 Remove specialization template<size_t N> struct formatter<const charT[N], charT>

Depends on D143845

[Mordante]

Retrigger CI.

[ldionne]

Nice cleanup!

libcxx/docs/ReleaseNotes.rst

66

Can you comment on how (if at all) this impacts users in the release note? Also, perhaps "Deprecations and Removals" would be a better section for this release note?

[Mordante]

libcxx/docs/ReleaseNotes.rst

66

I moved it. I don't expect anybody to be affected. I only affects people instantiating this specialization either directly or by inheritance. Direct specialization is extremely rare, typically formatters are used from the format functions. Inheriting itself is not rare, but the specialization for a basic_string, basic_string_view, or even const CharT* are more likely candidates for a base class for a custom string-like formatter.

If you want I can add more comment, but I think in this case it's not needed.

libcxx/docs/ReleaseNotes.rst

 
 - The ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro will not be honored anymore in LLVM 18.
   Please see the updated documentation about the safe libc++ mode and in particular the ``_LIBCPP_VERBOSE_ABORT``
   macro for details.
 
 API Changes
 -----------
 
+- The formatter specialization ``template<size_t N> struct formatter<const charT[N], charT>``
+  has been removed. Since libc++'s format library was marked experimental there
+  is no backwards compatibility option. This specialization has been removed
+  from the Standard since it was never used, the proper specialization to use
+  instead is ``template<size_t N> struct formatter<charT[N], charT>``.
+
 ABI Affecting Changes
 ---------------------
 
 Build System Changes
 --------------------

libcxx/docs/Status/Cxx2bIssues.csv

 "`3723 <https://wg21.link/LWG3723>`__","``priority_queue::push_range`` needs to ``append_range``","February 2023","","","|ranges|"
 "`3734 <https://wg21.link/LWG3734>`__","Inconsistency in ``inout_ptr`` and ``out_ptr`` for empty case","February 2023","","",""
 "`3772 <https://wg21.link/LWG3772>`__","``repeat_view``'s ``piecewise`` constructor is missing Postconditions","February 2023","","","|ranges|"
 "`3786 <https://wg21.link/LWG3786>`__","Flat maps' deduction guide needs to default ``Allocator`` to be useful","February 2023","","",""
 "`3803 <https://wg21.link/LWG3803>`__","``flat_foo`` constructors taking ``KeyContainer`` lack ``KeyCompare`` parameter","February 2023","","",""
 "`3810 <https://wg21.link/LWG3810>`__","CTAD for ``std::basic_format_args``","February 2023","","","|format|"
 "`3827 <https://wg21.link/LWG3827>`__","Deprecate ``<stdalign.h>`` and ``<stdbool.h>`` macros","February 2023","","",""
 "`3828 <https://wg21.link/LWG3828>`__","Sync ``intmax_t`` and ``uintmax_t`` with C2x","February 2023","","",""
-"`3833 <https://wg21.link/LWG3833>`__","Remove specialization ``template<size_t N> struct formatter<const charT[N], charT>``","February 2023","","","|format|"
+"`3833 <https://wg21.link/LWG3833>`__","Remove specialization ``template<size_t N> struct formatter<const charT[N], charT>``","February 2023","|Complete|","17.0","|format|"
 "`3836 <https://wg21.link/LWG3836>`__","``std::expected<bool, E1>`` conversion constructor ``expected(const expected<U, G>&)`` should take precedence over ``expected(U&&)`` with operator ``bool``","February 2023","","",""
 "`3843 <https://wg21.link/LWG3843>`__","``std::expected<T,E>::value() &`` assumes ``E`` is copy constructible","February 2023","","",""
 "`3847 <https://wg21.link/LWG3847>`__","``ranges::to`` can still return views","February 2023","","","|ranges|"
 "`3862 <https://wg21.link/LWG3862>`__","``basic_const_iterator``'s ``common_type`` specialization is underconstrained","February 2023","","",""
 "`3865 <https://wg21.link/LWG3865>`__","Sorting a range of ``pairs``","February 2023","","","|ranges|"
 "`3869 <https://wg21.link/LWG3869>`__","Deprecate ``std::errc`` constants related to UNIX STREAMS","February 2023","","",""
 "`3870 <https://wg21.link/LWG3870>`__","Remove ``voidify``","February 2023","","",""
 "`3871 <https://wg21.link/LWG3871>`__","Adjust note about ``terminate``","February 2023","","",""

libcxx/include/__format/formatter_string.h

struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<_CharT[_Size], _CharT>
     : public __formatter_string<_CharT> {
   using _Base = __formatter_string<_CharT>;
 
   _LIBCPP_HIDE_FROM_ABI auto format(_CharT __str[_Size], auto& __ctx) const -> decltype(__ctx.out()) {
     return _Base::format(basic_string_view<_CharT>(__str, _Size), __ctx);
   }
 };
 
-// Formatter const char[].
-template <__fmt_char_type _CharT, size_t _Size>
-struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<const _CharT[_Size], _CharT>
-    : public __formatter_string<_CharT> {
-  using _Base = __formatter_string<_CharT>;
-
-  _LIBCPP_HIDE_FROM_ABI auto format(const _CharT __str[_Size], auto& __ctx) const -> decltype(__ctx.out()) {
-    return _Base::format(basic_string_view<_CharT>(__str, _Size), __ctx);
-  }
-};
-
 // Formatter std::string.
 template <__fmt_char_type _CharT, class _Traits, class _Allocator>
 struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<basic_string<_CharT, _Traits, _Allocator>, _CharT>
     : public __formatter_string<_CharT> {
   using _Base = __formatter_string<_CharT>;
 
   _LIBCPP_HIDE_FROM_ABI auto format(const basic_string<_CharT, _Traits, _Allocator>& __str, auto& __ctx) const
       -> decltype(__ctx.out()) {

libcxx/include/__format/range_default_formatter.h

else if constexpr (requires { typename _Rp::key_type; }) {
     else
       // 2.2.2 Otherwise format_kind<R> is range_format::set.
       return range_format::set;
   } else
     // 2.3 Otherwise, format_kind<R> is range_format::sequence.
     return range_format::sequence;
 }();
 
-// This is a non-standard work-around to fix instantiation of
-//   formatter<const _CharT[N], _CharT>
-// const _CharT[N] satisfies the ranges::input_range concept.
-// remove_cvref_t<const _CharT[N]> is _CharT[N] so it does not satisfy the
-// requirement of the above specialization. Instead it will instantiate the
-// primary template, which is ill-formed.
-//
-// An alternative solution is to remove the offending formatter.
-//
-// https://godbolt.org/z/bqjhhaexx
-//
-// The removal is proposed in LWG3833, but use the work-around until the issue
-// has been adopted.
-// TODO FMT Implement LWG3833.
-template <class _CharT, size_t N>
-inline constexpr range_format format_kind<const _CharT[N]> = range_format::disabled;
-
 template <range_format _Kp, ranges::input_range _Rp, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter;
 
 // Required specializations
 
 template <ranges::input_range _Rp, class _CharT>
 struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __range_default_formatter<range_format::sequence, _Rp, _CharT> {
 private:

libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/formatter.const_char_array.pass.cpp

-//===----------------------------------------------------------------------===//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-// UNSUPPORTED: libcpp-has-no-incomplete-format
-// TODO FMT Evaluate gcc-12 status
-// UNSUPPORTED: gcc-12
-
-// <format>
-
-// C++23 the formatter is a debug-enabled specialization.
-// [format.formatter.spec]:
-// Each header that declares the template `formatter` provides the following
-// enabled specializations:
-// For each `charT`, the string type specializations
-//   template<size_t N> struct formatter<const charT[N], charT>;
-
-#include <format>
-#include <cassert>
-#include <concepts>
-#include <type_traits>
-
-#include "test_format_context.h"
-#include "test_macros.h"
-#include "make_string.h"
-
-#define STR(S) MAKE_STRING(CharT, S)
-#define SV(S) MAKE_STRING_VIEW(CharT, S)
-#define CSTR(S) MAKE_CSTRING(CharT, S)
-
-// This is based on the method found in
-// clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1-cxx20.cpp
-template <size_t N>
-struct Tester {
-  // This is not part of the real test, but is used the deduce the size of the input.
-  constexpr Tester(const char (&r)[N]) { __builtin_memcpy(text, r, N); }
-  char text[N];
-
-  // The size of the array shouldn't include the NUL character.
-  static const size_t size = N - 1;
-
-  template <class CharT>
-  void test(const std::basic_string<CharT>& expected,
-            const std::basic_string_view<CharT>& fmt) const {
-    using Str = const CharT[size];
-    std::basic_format_parse_context<CharT> parse_ctx{fmt};
-    std::formatter<Str, CharT> formatter;
-    static_assert(std::semiregular<decltype(formatter)>);
-
-    auto it = formatter.parse(parse_ctx);
-    assert(it == fmt.end() - (!fmt.empty() && fmt.back() == '}'));
-
-    std::basic_string<CharT> result;
-    auto out = std::back_inserter(result);
-    using FormatCtxT = std::basic_format_context<decltype(out), CharT>;
-
-    std::basic_string<CharT> buffer{text, text + N};
-    // Note not too found of this hack
-    Str* data = reinterpret_cast<Str*>(buffer.c_str());
-
-    FormatCtxT format_ctx =
-        test_format_context_create<decltype(out), CharT>(out, std::make_format_args<FormatCtxT>(*data));
-    formatter.format(*data, format_ctx);
-    assert(result == expected);
-  }
-
-  template <class CharT>
-  void test_termination_condition(const std::basic_string<CharT>& expected,
-                                  const std::basic_string<CharT>& f) const {
-    // The format-spec is valid if completely consumed or terminates at a '}'.
-    // The valid inputs all end with a '}'. The test is executed twice:
-    // - first with the terminating '}',
-    // - second consuming the entire input.
-    std::basic_string_view<CharT> fmt{f};
-    assert(fmt.back() == CharT('}') && "Pre-condition failure");
-
-    test(expected, fmt);
-    fmt.remove_suffix(1);
-    test(expected, fmt);
-  }
-};
-
-template <size_t N>
-Tester(const char (&)[N]) -> Tester<N>;
-
-template <Tester t, class CharT>
-void test_helper_wrapper(std::basic_string<CharT> expected,
-                         std::basic_string<CharT> fmt) {
-  t.test_termination_condition(expected, fmt);
-}
-
-#if TEST_STD_VER > 20
-template <class CharT>
-constexpr bool test_set_debug_format() {
-  std::formatter<const CharT[1], CharT> formatter;
-  LIBCPP_ASSERT(formatter.__parser_.__type_ == std::__format_spec::__type::__default);
-
-  formatter.set_debug_format();
-  LIBCPP_ASSERT(formatter.__parser_.__type_ == std::__format_spec::__type::__debug);
-
-  std::basic_string_view fmt = SV("s}");
-  std::basic_format_parse_context<CharT> parse_ctx{fmt};
-  formatter.parse(parse_ctx);
-  LIBCPP_ASSERT(formatter.__parser_.__type_ == std::__format_spec::__type::__string);
-
-  formatter.set_debug_format();
-  LIBCPP_ASSERT(formatter.__parser_.__type_ == std::__format_spec::__type::__debug);
-
-  return true;
-}
-#endif
-template <class CharT>
-void test_array() {
-  test_helper_wrapper<" azAZ09,./<>?">(STR(" azAZ09,./<>?"), STR("}"));
-
-  std::basic_string<CharT> s(CSTR("abc\0abc"), 7);
-  test_helper_wrapper<"abc\0abc">(s, STR("}"));
-
-  test_helper_wrapper<"world">(STR("world"), STR("}"));
-  test_helper_wrapper<"world">(STR("world"), STR("_>}"));
-
-  test_helper_wrapper<"world">(STR("   world"), STR(">8}"));
-  test_helper_wrapper<"world">(STR("___world"), STR("_>8}"));
-  test_helper_wrapper<"world">(STR("_world__"), STR("_^8}"));
-  test_helper_wrapper<"world">(STR("world___"), STR("_<8}"));
-
-  test_helper_wrapper<"world">(STR("world"), STR(".5}"));
-  test_helper_wrapper<"universe">(STR("unive"), STR(".5}"));
-
-  test_helper_wrapper<"world">(STR("%world%"), STR("%^7.7}"));
-  test_helper_wrapper<"universe">(STR("univers"), STR("%^7.7}"));
-
-#if TEST_STD_VER > 20
-  test_set_debug_format<CharT>();
-  static_assert(test_set_debug_format<CharT>());
-#endif
-}
-
-int main(int, char**) {
-  test_array<char>();
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  test_array<wchar_t>();
-#endif
-
-  return 0;
-}