[libcxx-commits] [PATCH] D133220: [libc++][ranges] fix errors on passing input iterator to `std::views::take`

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huixie90 added inline comments.


================
Comment at: libcxx/include/__ranges/take_view.h:229
 template <class _Iter, class _Sent, subrange_kind _Kind>
+  requires requires{typename subrange<_Iter>;}
 struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
----------------
var-const wrote:
> philnik wrote:
> > I think you said somewhere that GCC was OK without the requires clause. Could you check whether that's a GCC or Clang bug? (If you didn't, just ignore the comment)
> The wording is, with simplifications (see https://eel.is/c++draft/range.take#overview-2.2):
> ```
> ...if `T` models [...] `ranges​::​subrange`, then [...] `U` is a type determined as follows:
> [...]
> * otherwise, `T` is a specialization of `ranges​::​subrange`, and `U` is `ranges​::​subrange<iterator_­t<T>>`;
> ```
> Is this a defect in the standard, perhaps? The wording doesn't seem to take into account that `T` modeling `subrange` doesn't guarantee that `iterator_t<T>` models `subrange`. Might be worth asking on the reflector or writing an LWG issue (unless you don't think it's a defect, of course).
> 
> I'm also not sure if placing an additional constraint is the best solution here. Wouldn't returning `subrange<_Iter, _Sent>` reflect the intention of the standard better in this case (which is to avoid creating a `take_view` where its effect can easily be applied to the original type or a closely related type)?
> 
> Additionally, can you please check what other implementations are doing to inform our decision? From a brief glance, it seems like both GCC and MSVC would return `subrange<_Iter, _Sent>` in this case (though I could easily misread it, a Godbolt would be very helpful).
I don't think it is a defect in the standard. That paragraph has a condition "if `T` models `random_­access_­range`". In case of `random_­access_­range`, `subrange<Iter>` is always valid. But in this particular case of `input_iterator`, `subrange<Iter>` is invalid but it should not pass the "if `T` models `random_­access_­range`" condition, and should fall through https://eel.is/c++draft/range.take#overview-2.5

The standard is correct, in case of `random_access_range`, it should return `subrange<Iter>`, i.e. `subrange<Iter, Iter>`, because we need to return `subrange(it, it + n)` (both are iterators not sentinel). In case of `input_iterator`, it should simply fall under the last paragraph and return `take_view` (it should never return `subrange<Iter, Sent>`)



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  https://reviews.llvm.org/D133220/new/

https://reviews.llvm.org/D133220