[libc++][format] Addresses LWG3825.

[Mordante]

Summary

LWG3825 Missing compile-time argument id check in
basic_format_parse_context::next_arg_id

libcxx/docs/Status/Cxx2bIssues.csv

 "`3085 <https://wg21.link/LWG3085>`__","``char_traits::copy`` precondition too weak","February 2023","","",""
 "`3664 <https://wg21.link/LWG3664>`__","`LWG 3392 <https://wg21.link/LWG3392>`__ ``broke std::ranges::distance(a, a+3)``","February 2023","","","|ranges|"
 "`3720 <https://wg21.link/LWG3720>`__","Restrict the valid types of ``arg-id`` for width and precision in ``std-format-spec``","February 2023","","","|format|"
 "`3756 <https://wg21.link/LWG3756>`__","Is the ``std::atomic_flag`` class signal-safe?","February 2023","","",""
 "`3769 <https://wg21.link/LWG3769>`__","``basic_const_iterator::operator==`` causes infinite constraint recursion","February 2023","","","|spaceship|"
 "`3807 <https://wg21.link/LWG3807>`__","The feature test macro for ``ranges::find_last`` should be renamed","February 2023","","","|ranges|"
 "`3811 <https://wg21.link/LWG3811>`__","``views::as_const`` on ``ref_view<T>`` should return ``ref_view<const T>``","February 2023","","","|ranges|"
 "`3820 <https://wg21.link/LWG3820>`__","``cartesian_product_view::iterator::prev`` is not quite right","February 2023","","","|ranges|"
-"`3825 <https://wg21.link/LWG3825>`__","Missing compile-time argument ``id`` check in ``basic_format_parse_context::next_arg_id``","February 2023","","","|format|"
+"`3825 <https://wg21.link/LWG3825>`__","Missing compile-time argument ``id`` check in ``basic_format_parse_context::next_arg_id``","February 2023","|Complete|","17.0","|format|"
 "`3204 <https://wg21.link/LWG3204>`__","``sub_match::swap`` only swaps the base class","February 2023","|Complete|","17.0",""
 "`3733 <https://wg21.link/LWG3733>`__","``ranges::to`` misuses ``cpp17-input-iterator``","February 2023","","","|ranges|"
 "`3742 <https://wg21.link/LWG3742>`__","``deque::prepend_range`` needs to permute","February 2023","","","|ranges|"
 "`3790 <https://wg21.link/LWG3790>`__","`P1467 <https://wg21.link/P1467>`__ accidentally changed ``nexttoward``'s signature","February 2023","","",""
 "`3819 <https://wg21.link/LWG3819>`__","``reference_meows_from_temporary`` should not use ``is_meowible``","February 2023","","",""
 "`3821 <https://wg21.link/LWG3821>`__","``uses_allocator_construction_args`` should have overload for ``pair-like``","February 2023","","",""
 "`3834 <https://wg21.link/LWG3834>`__","Missing ``constexpr`` for ``std::intmax_t`` math functions in ``<cinttypes>``","February 2023","","",""
 "`3839 <https://wg21.link/LWG3839>`__","``range_formatter``'s ``set_separator``, ``set_brackets``, and ``underlying`` functions should be ``noexcept``","February 2023","|Complete|","17.0","|format|"

libcxx/include/__format/format_parse_context.h

public:
   }
 
   _LIBCPP_HIDE_FROM_ABI constexpr size_t next_arg_id() {
     if (__indexing_ == __manual)
       std::__throw_format_error("Using automatic argument numbering in manual argument numbering mode");
 
     if (__indexing_ == __unknown)
       __indexing_ = __automatic;
+
+    // Throws an exception to make the expression a non core constant
+    // expression as required by:
+    // [format.parse.ctx]/8
+    //   Remarks: Let cur-arg-id be the value of next_arg_id_ prior to this
+    //   call. Call expressions where cur-arg-id >= num_args_ is true are not
+    //   core constant expressions (7.7 [expr.const]).
+    // Note: the Throws clause [format.parse.ctx]/9 doesn't specify the
+    // behavior when id >= num_args_.
+    if (is_constant_evaluated() && __next_arg_id_ >= __num_args_)
+      std::__throw_format_error("Argument index outside the valid range");
+
     return __next_arg_id_++;
   }
   _LIBCPP_HIDE_FROM_ABI constexpr void check_arg_id(size_t __id) {
     if (__indexing_ == __automatic)
       std::__throw_format_error("Using manual argument numbering in automatic argument numbering mode");
 
     if (__indexing_ == __unknown)
       __indexing_ = __manual;

libcxx/test/std/utilities/format/format.formatter/format.parse.ctx/next_arg_id.pass.cpp

 
 #include <cassert>
 #include <cstring>
 #include <string_view>
 
 #include "test_macros.h"
 
 constexpr bool test() {
-  std::format_parse_context context("");
+  std::format_parse_context context("", 10);
   for (size_t i = 0; i < 10; ++i)
     assert(i == context.next_arg_id());
 
   return true;
 }
 
 void test_exception() {
   std::format_parse_context context("", 1);

libcxx/test/std/utilities/format/format.formatter/format.parse.ctx/next_arg_id.verify.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
+
+// constexpr size_t next_arg_id()
+
+#include <format>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  // [format.parse.ctx]/8
+  // Let cur-arg-id be the value of next_arg_id_ prior to this call. Call
+  // expressions where cur-arg-id >= num_args_ is true are not core constant
+  // expressions (7.7 [expr.const]).
+  std::format_parse_context context("", 0);
+  context.next_arg_id();
+
+  return true;
+}
+
+int main(int, char**) {
+  // expected-error-re@+1 {{{{(static_assert|static assertion)}} expression is not an integral constant expression}}
+  static_assert(test());
+
+  return 0;
+}