[libcxx-commits] [libcxx] [libc++][format] LWG4106: `basic_format_args` should not be default-constructible (PR #97250)

[A. Jiang via libcxx-commits]

https://github.com/frederick-vs-ja created https://github.com/llvm/llvm-project/pull/97250

See [LWG4106](https://cplusplus.github.io/LWG/issue4106) and [P3341R0](https://wg21.link/p3341r0).

The test coverage for the empty state of `basic_format_args` in `get.pass.cpp` is to be completely removed, because the non-default-constructibility is covered in `ctor.pass.cpp`.

>From 80e556adc35758a1700f101908e7929f796294e8 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34 at live.cn>
Date: Mon, 1 Jul 2024 10:21:55 +0800
Subject: [PATCH] [libc++][format] Implement LWG4106

---
 libcxx/include/__format/format_args.h                    | 2 --
 .../format/format.arguments/format.args/ctor.pass.cpp    | 9 ++-------
 .../format/format.arguments/format.args/get.pass.cpp     | 5 -----
 3 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/libcxx/include/__format/format_args.h b/libcxx/include/__format/format_args.h
index a5fde36a298174..07923570f38930 100644
--- a/libcxx/include/__format/format_args.h
+++ b/libcxx/include/__format/format_args.h
@@ -28,8 +28,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Context>
 class _LIBCPP_TEMPLATE_VIS basic_format_args {
 public:
-  basic_format_args() noexcept = default;
-
   template <class... _Args>
   _LIBCPP_HIDE_FROM_ABI basic_format_args(const __format_arg_store<_Context, _Args...>& __store) noexcept
       : __size_(sizeof...(_Args)) {
diff --git a/libcxx/test/std/utilities/format/format.arguments/format.args/ctor.pass.cpp b/libcxx/test/std/utilities/format/format.arguments/format.args/ctor.pass.cpp
index c0575c545bde31..bb542a8e63ecf6 100644
--- a/libcxx/test/std/utilities/format/format.arguments/format.args/ctor.pass.cpp
+++ b/libcxx/test/std/utilities/format/format.arguments/format.args/ctor.pass.cpp
@@ -9,12 +9,12 @@
 
 // <format>
 
-// basic_format_args() noexcept;
 // template<class... Args>
 //   basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;
 
 #include <format>
 #include <cassert>
+#include <type_traits>
 
 #include "test_macros.h"
 
@@ -24,12 +24,7 @@ void test() {
   char c        = 'c';
   nullptr_t p   = nullptr;
   using Context = std::basic_format_context<CharT*, CharT>;
-  {
-    ASSERT_NOEXCEPT(std::basic_format_args<Context>{});
-
-    std::basic_format_args<Context> format_args{};
-    assert(!format_args.get(0));
-  }
+  static_assert(!std::is_default_constructible_v<std::basic_format_args<Context>>);
   {
     auto store = std::make_format_args<Context>(i);
     ASSERT_NOEXCEPT(std::basic_format_args<Context>{store});
diff --git a/libcxx/test/std/utilities/format/format.arguments/format.args/get.pass.cpp b/libcxx/test/std/utilities/format/format.arguments/format.args/get.pass.cpp
index c590cebf48accd..af8d7eff6f6b8d 100644
--- a/libcxx/test/std/utilities/format/format.arguments/format.args/get.pass.cpp
+++ b/libcxx/test/std/utilities/format/format.arguments/format.args/get.pass.cpp
@@ -82,11 +82,6 @@ void test_string_view(From value) {
 template <class CharT>
 void test() {
   using Context = std::basic_format_context<CharT*, CharT>;
-  {
-    const std::basic_format_args<Context> format_args{};
-    ASSERT_NOEXCEPT(format_args.get(0));
-    assert(!format_args.get(0));
-  }
 
   using char_type = typename Context::char_type;
   std::basic_string<char_type> empty;