[libcxx-commits] [libcxx] 1821bc1 - [libc++] Fix the handling of `views::take` for `iota_view` (#75683)

[via libcxx-commits]

Author: A. Jiang
Date: 2023-12-18T15:41:14-05:00
New Revision: 1821bc1e969bd2d0887fe115ae371506adc1a8be

URL: https://github.com/llvm/llvm-project/commit/1821bc1e969bd2d0887fe115ae371506adc1a8be
DIFF: https://github.com/llvm/llvm-project/commit/1821bc1e969bd2d0887fe115ae371506adc1a8be.diff

LOG: [libc++] Fix the handling of `views::take` for `iota_view` (#75683)

Currently, when libc++'s views::take specially handles an iota_view, the
addition is done after dereferencing the beginning iterator. However, in
[range.take.overview]/2.3, the addition is done before the dereferencing,
which means that the standard requires the returned iota_view to have
the same W and Bound type in such cases.

This patch fixes that, and also fixes a test that was testing the 
incorrect behavior.

Fixes #75611

Added: 
    

Modified: 
    libcxx/include/__ranges/take_view.h
    libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp

Removed: 
    


################################################################################
diff  --git a/libcxx/include/__ranges/take_view.h b/libcxx/include/__ranges/take_view.h
index 4fd1d8b9ab8377..83ed5ca0ebd390 100644
--- a/libcxx/include/__ranges/take_view.h
+++ b/libcxx/include/__ranges/take_view.h
@@ -276,23 +276,31 @@ struct __fn {
   }
 
   // [range.take.overview]: the `iota_view` case.
+  // clang-format off
   template <class _Range,
             convertible_to<range_
diff erence_t<_Range>> _Np,
             class _RawRange = remove_cvref_t<_Range>,
             class _Dist     = range_
diff erence_t<_Range>>
-    requires(!__is_empty_view<_RawRange> && random_access_range<_RawRange> && sized_range<_RawRange> &&
-             __is_iota_specialization<_RawRange>)
-  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto
-  operator()(_Range&& __rng, _Np&& __n) const noexcept(noexcept(ranges::iota_view(
-      *ranges::begin(__rng), *ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))))
-      -> decltype(ranges::iota_view(
-          // Note: deliberately not forwarding `__rng` to guard against double moves.
-          *ranges::begin(__rng),
-          *ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))) {
-    return ranges::iota_view(*ranges::begin(__rng),
-                             *ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)));
-  }
-  // clang-format off
+    requires (!__is_empty_view<_RawRange> &&
+              random_access_range<_RawRange> &&
+              sized_range<_RawRange> &&
+              __is_iota_specialization<_RawRange>)
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
+  constexpr auto operator()(_Range&& __rng, _Np&& __n) const
+    noexcept(noexcept(ranges::iota_view(
+                              *ranges::begin(__rng),
+                              *(ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
+                              )))
+    -> decltype(      ranges::iota_view(
+                              // Note: deliberately not forwarding `__rng` to guard against double moves.
+                              *ranges::begin(__rng),
+                              *(ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
+                              ))
+    { return          ranges::iota_view(
+                              *ranges::begin(__rng),
+                              *(ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
+                              ); }
+
 #if _LIBCPP_STD_VER >= 23
   // [range.take.overview]: the `repeat_view` "_RawRange models sized_range" case.
   template <class _Range,

diff  --git a/libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp
index 8ffac8d8fab153..bb5b5f5ff4909a 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp
+++ b/libcxx/test/std/ranges/range.adaptors/range.take/adaptor.pass.cpp
@@ -172,9 +172,8 @@ constexpr bool test() {
   // `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 
diff erent because it has to be able to hold
-    // the `range_
diff erence_t` of the input `iota_view`.
-    using Result = std::ranges::iota_view<int, std::ranges::range_
diff erence_t<decltype(iota)>>;
+    // The second template argument of the resulting `iota_view` is same as the first.
+    using Result                               = std::ranges::iota_view<int, int>;
     std::same_as<Result> decltype(auto) result = iota | std::views::take(3);
     assert(result.size() == 3);
   }