> We will reference C23 soon, so it is.

There seems to be a misunderstanding here. Just because C++26 is based
on C23 doesn't mean that it inherits every language feature from C23.
It just means that the references to the document are updated, which
is purely editorial, and that more stuff from the C standard library
is acknowledged to exist in C++.

By the same logic, C++ should have compound literals, but I'm not
seeing those in C++ despite us being based on C99 for quite some time.

That being said, I am working on getting _BitInt into the C++
standard, but that is a ton of effort.

I also think that _BitInt may be essential to this proposal,
unfortunately. It's hard to see how parse_float could be constexpr if
it operated on std::span<const std::byte>. This would require
reinterpret_cast. On the other hand, _BitInt(128) would be enough
mantissa bits even for std::float128_t.