[libc++][ranges] fix errors on passing input iterator to `std::views::take`

[huixie90]

Summary

In the implementation of std::views::take, it uses subrange<Iter> as part of the return type. But in case of input iterator, subrange<Iter> can be ill-formed

[philnik]

Could you add [libc++] to the commit message?
LGTM % nits. Thanks for the quick fix; sorry that I didn't review it earlier.

libcxx/include/__ranges/take_view.h

229

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)

libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp

130

clang-format seems to be quite unhappy here. Could you file a bug and fix the formatting for now?

199

Nit: Use CTAD?

200

The [[maybe_unused]] seems to be unnecessary.

[var-const]

libcxx/include/__ranges/take_view.h

229

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).

libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp

107

Nit: it's better to commit reformatting in a separate NFC patch, it makes reviewing easier. But feel free to keep in this patch if splitting it would be too much of a hassle.

130

+1.

200

+1.

[huixie90]

addressed comments

[huixie90]

libcxx/include/__ranges/take_view.h

229

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>)

[var-const]

libcxx/include/__ranges/take_view.h

229

You're right -- sorry, I missed the random_access_range part. In that case, it looks like this is the right fix.

libcxx/include/__ranges/take_view.h

 };
 
 template <class _CharT, class _Traits>
 struct __passthrough_type<basic_string_view<_CharT, _Traits>> {
   using type = basic_string_view<_CharT, _Traits>;
 };
 
 template <class _Iter, class _Sent, subrange_kind _Kind>
+  requires requires{typename subrange<_Iter>;}
 struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
   using type = subrange<_Iter>;
 };
 
 template <class _Tp>
 using __passthrough_type_t = typename __passthrough_type<_Tp>::type;
 
 struct __fn {

libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp

 #include <cassert>
 #include <concepts>
 #include <span>
 #include <string_view>
 #include <utility>
 #include "test_iterators.h"
 
 template <class View, class T>
-concept CanBePiped = requires (View&& view, T&& t) {
+concept CanBePiped =
+    requires(View&& view, T&& t) {
       { std::forward<View>(view) | std::forward<T>(t) };
     };
 
 struct SizedView : std::ranges::view_base {
   int* begin_ = nullptr;
-  int* end_ = nullptr;
+  int* end_   = nullptr;
   constexpr SizedView(int* begin, int* end) : begin_(begin), end_(end) {}
 
   constexpr auto begin() const { return forward_iterator<int*>(begin_); }
   constexpr auto end() const { return sized_sentinel<forward_iterator<int*>>(forward_iterator<int*>(end_)); }
 };
 static_assert(std::ranges::forward_range<SizedView>);
 static_assert(std::ranges::sized_range<SizedView>);
 static_assert(std::ranges::view<SizedView>);
 
 template <class T>
 constexpr void test_small_range(const T& input) {
   constexpr int N = 100;
-  auto size = std::ranges::size(input);
+  auto size       = std::ranges::size(input);
 
   auto result = input | std::views::take(N);
   assert(size < N);
   assert(result.size() == size);
 }
 
 constexpr bool test() {
   constexpr int N = 8;
-  int buf[N] = {1, 2, 3, 4, 5, 6, 7, 8};
+  int buf[N]      = {1, 2, 3, 4, 5, 6, 7, 8};
 
   // Test that `std::views::take` is a range adaptor.
   {
     using SomeView = SizedView;
 
     // Test `view | views::take`
     {
       SomeView view(buf, buf + N);
       std::same_as<std::ranges::take_view<SomeView>> decltype(auto) result = view | std::views::take(3);
       assert(result.base().begin_ == buf);
       assert(result.base().end_ == buf + N);
       assert(result.size() == 3);
     }
 
     // Test `adaptor | views::take`
     {
       SomeView view(buf, buf + N);
-      auto f = [](int i) { return i; };
+      auto f             = [](int i) { return i; };
       auto const partial = std::views::transform(f) | std::views::take(3);
 
       using Result = std::ranges::take_view<std::ranges::transform_view<SomeView, decltype(f)>>;
       std::same_as<Result> decltype(auto) result = partial(view);
       assert(result.base().base().begin_ == buf);
       assert(result.base().base().end_ == buf + N);
       assert(result.size() == 3);
     }
 
     // Test `views::take | adaptor`
     {
       SomeView view(buf, buf + N);
-      auto f = [](int i) { return i; };
+      auto f             = [](int i) { return i; };
       auto const partial = std::views::take(3) | std::views::transform(f);
 
       using Result = std::ranges::transform_view<std::ranges::take_view<SomeView>, decltype(f)>;
       std::same_as<Result> decltype(auto) result = partial(view);
       assert(result.base().base().begin_ == buf);
       assert(result.base().base().end_ == buf + N);
       assert(result.size() == 3);
     }
 
     // Check SFINAE friendliness
     {
-      struct NotAView { };
+      struct NotAView {};
       static_assert(!std::is_invocable_v<decltype(std::views::take)>);
       static_assert(!std::is_invocable_v<decltype(std::views::take), NotAView, int>);
-      static_assert( CanBePiped<SomeView&,   decltype(std::views::take(3))>);
-      static_assert( CanBePiped<int(&)[10],  decltype(std::views::take(3))>);
+      static_assert(CanBePiped<SomeView&, decltype(std::views::take(3))>);
+      static_assert(CanBePiped<int(&)[10], decltype(std::views::take(3))>);
       static_assert(!CanBePiped<int(&&)[10], decltype(std::views::take(3))>);
-      static_assert(!CanBePiped<NotAView,    decltype(std::views::take(3))>);
+      static_assert(!CanBePiped<NotAView, decltype(std::views::take(3))>);
 
-      static_assert(!CanBePiped<SomeView&,   decltype(std::views::take(/*n=*/NotAView{}))>);
+      static_assert(!CanBePiped<SomeView&, decltype(std::views::take(/*n=*/NotAView{}))>);
     }
   }
 
-  {
-    static_assert(std::same_as<decltype(std::views::take), decltype(std::ranges::views::take)>);
-  }
+  { static_assert(std::same_as<decltype(std::views::take), decltype(std::ranges::views::take)>); }
 
   // `views::take(empty_view, n)` returns an `empty_view`.
   {
-    using Result = std::ranges::empty_view<int>;
+    using Result                                                = std::ranges::empty_view<int>;
     [[maybe_unused]] std::same_as<Result> decltype(auto) result = std::views::empty<int> | std::views::take(3);
   }
 
   // `views::take(span, n)` returns a `span`.
   {
     std::span<int> s(buf);
     std::same_as<decltype(s)> decltype(auto) result = s | std::views::take(3);
     assert(result.size() == 3);
   }
 
   // `views::take(span, n)` returns a `span` with a dynamic extent, regardless of the input `span`.
   {
     std::span<int, 8> s(buf);
     std::same_as<std::span<int, std::dynamic_extent>> decltype(auto) result = s | std::views::take(3);
     assert(result.size() == 3);
   }
 
   // `views::take(string_view, n)` returns a `string_view`.
-  {
-    {
-      std::string_view sv = "abcdef";
-      std::same_as<decltype(sv)> decltype(auto) result = sv | std::views::take(3);
-      assert(result.size() == 3);
-    }
+  {{std::string_view sv = "abcdef";
 
-    {
-      std::u32string_view sv = U"abcdef";
-      std::same_as<decltype(sv)> decltype(auto) result = sv | std::views::take(3);
-      assert(result.size() == 3);
-    }
-  }
+  std::u32string_view sv                           = U"abcdef";
 
-  // `views::take(subrange, n)` returns a `subrange`.
-  {
-    auto subrange = std::ranges::subrange(buf, buf + N);
-    using Result = std::ranges::subrange<int*>;
-    std::same_as<Result> decltype(auto) result = subrange | std::views::take(3);
-    assert(result.size() == 3);
-  }
+  auto subrange                              = std::ranges::subrange(buf, buf + N);
+  using Result                               = std::ranges::subrange<int*>;
 
-  // `views::take(subrange, n)` doesn't return a `subrange` if it's not a random access range.
-  {
-    SizedView v(buf, buf + N);
-    auto subrange = std::ranges::subrange(v.begin(), v.end());
 
-    using Result = std::ranges::take_view<std::ranges::subrange<forward_iterator<int*>,
-        sized_sentinel<forward_iterator<int*>>>>;
-    std::same_as<Result> decltype(auto) result = subrange | std::views::take(3);
-    assert(result.size() == 3);
-  }
+  using Result =
+      std::ranges::take_view<std::ranges::subrange<forward_iterator<int*>, sized_sentinel<forward_iterator<int*>>>>;
 
-  // `views::take(subrange, n)` returns a `subrange` with all default template arguments.
-  {
-    std::ranges::subrange<int*, sized_sentinel<int*>, std::ranges::subrange_kind::sized> subrange;
 
-    using Result = std::ranges::subrange<int*, int*, std::ranges::subrange_kind::sized>;
-    [[maybe_unused]] std::same_as<Result> decltype(auto) result = subrange | std::views::take(3);
-  }
 
-  // `views::take(iota_view, n)` returns an `iota_view`.
-  {
-    auto iota = std::views::iota(1, 8);
-    // The second template argument of the resulting `iota_view` is different because it has to be able to hold
-    // the `range_difference_t` of the input `iota_view`.
-    using Result = std::ranges::iota_view<int, std::ranges::range_difference_t<decltype(iota)>>;
-    std::same_as<Result> decltype(auto) result = iota | std::views::take(3);
-    assert(result.size() == 3);
-  }
 
-  // When the size of the input range `s` is shorter than `n`, only `s` elements are taken.
-  {
-    test_small_range(std::span(buf));
-    test_small_range(std::string_view("abcdef"));
-    test_small_range(std::ranges::subrange(buf, buf + N));
-    test_small_range(std::views::iota(1, 8));
-  }
 
-  // Test that it's possible to call `std::views::take` with any single argument as long as the resulting closure is
-  // never invoked. There is no good use case for it, but it's valid.
-  {
-    struct X { };
-    [[maybe_unused]] auto partial = std::views::take(X{});
-  }
+  struct X {};
 
-  return true;
+// Test when `subrange<Iter>` is not well formed
+{
+  int input[] = {1, 2, 3};
+  using Iter  = cpp20_input_iterator<int*>;
+  using Sent  = sentinel_wrapper<Iter>;
+  std::ranges::subrange<Iter, Sent> r{Iter{input}, Sent{Iter{input + 3}}};
+  [[maybe_unused]] auto tv = std::views::take(std::move(r), 1);
+  auto it = tv.begin();
+  assert(*it == 1);
+  ++it;
+  assert(it == tv.end());
+}
+
 }
 
 int main(int, char**) {
   test();
   static_assert(test());
 
   return 0;
 }