[libc++][format] Addresses LWG3810.

[Mordante]

Summary

LWG3810 CTAD for std::basic_format_args

[philnik]

LGTM.

libcxx/include/__format/format_args.h

76

Was there no test for the CTAD support before?

[Mordante]

Thanks for the review!

libcxx/include/__format/format_args.h

76

No that is the "silence the compiler when we want implicit CTAD" version. We don't have tests for these.

[philnik]

libcxx/include/__format/format_args.h

76

I was asked by Louis to add one for the one I added for move_iterator(?). Weird that we don't have tests for all of them.

[Mordante]

libcxx/include/__format/format_args.h

76

The original CTAD was added in D133535, but no test. I see some implicit CTAD tests added in that revision.
@ldionne should these test have been added for all types with the CTAD macro?

[ldionne]

libcxx/include/__format/format_args.h

76

Yes, tests should have been added in D133535 for all public types where I added the _LIBCPP_CTAD_SUPPORTED_FOR_TYPE macro. I just checked and it seems like I only missed this one, all the other ones seem to have tests.

[Mordante]

libcxx/include/__format/format_args.h

76

Thanks, it seems the LWG issue was very useful ;-)

libcxx/docs/Status/Cxx2bIssues.csv

 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","February 2023","|Complete|","15.0",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","February 2023","|Complete|","14.0",""
 "`3655 <https://wg21.link/LWG3655>`__","The ``INVOKE`` operation and union types","February 2023","","",""
 "`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|"
+"`3810 <https://wg21.link/LWG3810>`__","CTAD for ``std::basic_format_args``","February 2023","|Complete|","17.0","|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|"
 "`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|"

libcxx/include/__format/format_args.h

private:
   union {
     struct {
       const __basic_format_arg_value<_Context>* __values_;
       uint64_t __types_;
     };
     const basic_format_arg<_Context>* __args_;
   };
 };
-_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(basic_format_args);
+
+template <class _Context, class... _Args>
+basic_format_args(__format_arg_store<_Context, _Args...>) -> basic_format_args<_Context>;
 
 #endif //_LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_FORMAT_ARGS_H

libcxx/test/std/utilities/format/format.arguments/format.args/ctad.compile.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
+
+// <format>
+
+// template<class Context, class... Args>
+//   basic_format_args(format-arg-store<Context, Args...>) -> basic_format_args<Context>;
+
+#include <concepts>
+#include <format>
+
+#include "test_macros.h"
+
+void test() {
+  // Note the Standard way to create a format-arg-store is by using make_format_args.
+  static_assert(std::same_as<decltype(std::basic_format_args(std::make_format_args(42))),
+                             std::basic_format_args<std::format_context>>);
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  static_assert(std::same_as<decltype(std::basic_format_args(std::make_wformat_args(42))),
+                             std::basic_format_args<std::wformat_context>>);
+
+#endif
+}