[libc++] `std::format` reads past the 0 terminator in `char[N]` arrays

[Fulgen301]

Reproducer

If one passes a fixed size char - array containing a 0-terminated string to std::format, the entire array minus one character is added to the result, instead of just the part until the null terminator. This not only seemingly violates [format.formatter.spec]/2.2, but also doesn't match the behavior of MS-STL or libstdc++. (And only formatting 49 characters out of a 50 character array seems buggy anyway.)

The issue is reproducible with all versions of libc++ that shipped with clang since <format> was added, including current trunk on godbolt.

[frederick-vs-ja]

See here:

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

Lines 104 to 116 in 37143fe

	// Char pointer
	template <class _Context, class _Tp>
	requires(same_as<typename _Context::char_type*, _Tp> || same_as<const typename _Context::char_type*, _Tp>)
	consteval __arg_t __determine_arg_t() {
	return __arg_t::__const_char_type_ptr;
	}
	// Char array
	template <class _Context, class _Tp>
	requires(is_array_v<_Tp> && same_as<_Tp, typename _Context::char_type[extent_v<_Tp>]>)
	consteval __arg_t __determine_arg_t() {
	return __arg_t::__string_view;
	}

Per [format.arg]/6, char arrays should be converted to char pointers first.

CC @mordante