[gcc r14-10537] libstdc++: Remove std::basic_format_args default constructor (LWG 4106)

[Jonathan Wakely]

https://gcc.gnu.org/g:a78480c4e92f6aa2e4b736fd393037df2267b7f8

commit r14-10537-ga78480c4e92f6aa2e4b736fd393037df2267b7f8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jul 25 13:52:12 2024 +0100

    libstdc++: Remove std::basic_format_args default constructor (LWG 4106)
    
    There's no valid use case for default constructing this type, so the
    committee approved removing the default constructor.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (basic_format_args): Remove default
            constructor, as per LWG 4106.
            * testsuite/std/format/arguments/args.cc: Check it isn't default
            constructible.
    
    (cherry picked from commit 5be55447e256302324f38f04316a437909ae5847)

Diff:
---
 libstdc++-v3/include/std/format                     | 2 --
 libstdc++-v3/testsuite/std/format/arguments/args.cc | 4 ++++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 16cee0d3c74d..8f6a82a1fd41 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -3667,8 +3667,6 @@ namespace __format
 	{ return {_Format_arg::template _S_to_enum<_Args>()...}; }
 
     public:
-      basic_format_args() noexcept = default;
-
       template<typename... _Args>
 	basic_format_args(const _Store<_Args...>& __store) noexcept;
 
diff --git a/libstdc++-v3/testsuite/std/format/arguments/args.cc b/libstdc++-v3/testsuite/std/format/arguments/args.cc
index eba129ff8944..16ca71caecbe 100644
--- a/libstdc++-v3/testsuite/std/format/arguments/args.cc
+++ b/libstdc++-v3/testsuite/std/format/arguments/args.cc
@@ -3,6 +3,10 @@
 #include <format>
 #include <testsuite_hooks.h>
 
+// LWG 4106. basic_format_args should not be default-constructible
+static_assert( ! std::is_default_constructible_v<std::format_args> );
+static_assert( ! std::is_default_constructible_v<std::wformat_args> );
+
 template<typename Ctx, typename T>
 bool equals(std::basic_format_arg<Ctx> fmt_arg, T expected) {
   return std::visit_format_arg([=](auto arg_val) {