Well...but nothing in the paper says that functions having a narrow contract can be marked as conditional noexcept.

And std::begin is similar to std::cbegin, it simply delegates to c.begin(), which is allowed to throw, but std::begin is not marked as conditional noexcept. Similar examples are std::end, std::size, etc.

On Mon, Aug 29, 2022 at 10:22 AM blacktea hamburger via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
I have noticed that some standard functions having a wide contract such as functions in [iterator.range] conditionally do not throw exceptions, but they are not marked as conditionally noexcept.


It is described in N3279: [2011]

Note that there's also a later paper by Nico Josuttis [2018]:
I don't think any differences are relevant to your question, but just FYI.
 

But there are some functions that do not obey this guideline such as std::cbegin. And I did not find a reason in this paper.

Can you clarify what you mean?  Looking at cppreference,
I see that
(1) std::cbegin in fact has a narrow contract; it simply delegates to std::begin(x), which is allowed to throw.
(2) std::cbegin is marked conditionally noexcept.

So, I'm not sure what you're asking.

–Arthur