visit_format_arg hiding of 128-bit ints is still broken

[t3nsor]

libstdc++ fixed a bug where 128-bit ints are visible to visitors passed to std::visit_format_arg. There was an attempt to do the same in libc++ but it is insufficient. Although 128-bit int types are now wrapped in handles before being passed to the user-provided visitor, the current libc++ still requires overload resolution to succeed for passing 128-bit int types directly to the user-provided visitor, as can be seen in this godbolt link. https://godbolt.org/z/r5vj31M3v

[frederick-vs-ja]

Looks like that these places are problematic.

llvm-project/libcxx/include/__format/format_arg.h

Lines 117 to 121 in 8857bef

	# if _LIBCPP_HAS_INT128
	return std::invoke(std::forward<_Visitor>(__vis), __arg.__value_.__i128_);
	# else
	__libcpp_unreachable();
	# endif

llvm-project/libcxx/include/__format/format_arg.h

Lines 127 to 131 in 8857bef

	# if _LIBCPP_HAS_INT128
	return std::invoke(std::forward<_Visitor>(__vis), __arg.__value_.__u128_);
	# else
	__libcpp_unreachable();
	# endif

llvm-project/libcxx/include/__format/format_arg.h

Lines 168 to 172 in 8857bef

	# if _LIBCPP_HAS_INT128
	return std::invoke_r<_Rp>(std::forward<_Visitor>(__vis), __arg.__value_.__i128_);
	# else
	__libcpp_unreachable();
	# endif

llvm-project/libcxx/include/__format/format_arg.h

Lines 178 to 182 in 8857bef

	# if _LIBCPP_HAS_INT128
	return std::invoke_r<_Rp>(std::forward<_Visitor>(__vis), __arg.__value_.__u128_);
	# else
	__libcpp_unreachable();
	# endif

I guess it's better to change __visit_format_arg.

[frederick-vs-ja]

I found a similar issue. The following program should compile, but libc++ rejects it because a handle prvalue is visited. Per the current specification, visitors can only take cv-unqualified lvalues in these functions.

Godbolt link

#include <format>

int main() {
  int n = 42;
  std::basic_format_args<std::format_context> format_args{std::make_format_args<std::format_context>(n)};

  auto lvalue_visitor = [](auto&){};
  std::visit_format_arg(lvalue_visitor, format_args.get(0));
  format_args.get(0).visit(lvalue_visitor);
  format_args.get(0).visit<void>(lvalue_visitor);
}

I'll handle this in a later PR.