[libc++][ranges] Implement LWG4053 and LWG4054 (#88612)

Implement
- LWG4053 Unary call to `std::views::repeat` does not decay the argument
- LWG4054 Repeating a `repeat_view` should repeat the view

Signed-off-by: yronglin <[email protected]>

Author: yronglin

libcxx/docs/Status/Cxx2cIssues.csv

@@ -59,8 +59,8 @@
 "`4038 <https://wg21.link/LWG4038>`__","``std::text_encoding::aliases_view`` should have constexpr iterators","Tokyo March 2024","","",""
 "`4043 <https://wg21.link/LWG4043>`__","""ASCII"" is not a registered character encoding","Tokyo March 2024","|Nothing To Do|","",""
 "`4045 <https://wg21.link/LWG4045>`__","``tuple`` can create dangling references from ``tuple-like``","Tokyo March 2024","","",""
-"`4053 <https://wg21.link/LWG4053>`__","Unary call to ``std::views::repeat`` does not decay the argument","Tokyo March 2024","","","|ranges|"
-"`4054 <https://wg21.link/LWG4054>`__","Repeating a ``repeat_view`` should repeat the view","Tokyo March 2024","","","|ranges|"
+"`4053 <https://wg21.link/LWG4053>`__","Unary call to ``std::views::repeat`` does not decay the argument","Tokyo March 2024","|Complete|","19.0","|ranges|"
+"`4054 <https://wg21.link/LWG4054>`__","Repeating a ``repeat_view`` should repeat the view","Tokyo March 2024","|Complete|","19.0","|ranges|"
 "","","","","",""
 "`3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","16.0",""
 "XXXX","","The sys_info range should be affected by save","Not Yet Adopted","|Complete|","19.0"

libcxx/include/__ranges/repeat_view.h

@@ -22,6 +22,7 @@
 #include <__ranges/iota_view.h>
 #include <__ranges/movable_box.h>
 #include <__ranges/view_interface.h>
+#include <__type_traits/decay.h>
 #include <__type_traits/is_object.h>
 #include <__type_traits/make_unsigned.h>
 #include <__type_traits/remove_cv.h>
@@ -127,8 +128,8 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS repeat_view : public view_interface<r
   _LIBCPP_NO_UNIQUE_ADDRESS _Bound __bound_ = _Bound();
 };
 
-template <class _Tp, class _Bound>
-repeat_view(_Tp, _Bound) -> repeat_view<_Tp, _Bound>;
+template <class _Tp, class _Bound = unreachable_sentinel_t>
+repeat_view(_Tp, _Bound = _Bound()) -> repeat_view<_Tp, _Bound>;
 
 // [range.repeat.iterator]
 template <move_constructible _Tp, semiregular _Bound>
@@ -230,9 +231,9 @@ namespace __repeat {
 struct __fn {
   template <class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Tp&& __value)
-    noexcept(noexcept(ranges::repeat_view(std::forward<_Tp>(__value))))
-    -> decltype(      ranges::repeat_view(std::forward<_Tp>(__value)))
-    { return          ranges::repeat_view(std::forward<_Tp>(__value)); }
+    noexcept(noexcept(ranges::repeat_view<decay_t<_Tp>>(std::forward<_Tp>(__value))))
+    -> decltype(      ranges::repeat_view<decay_t<_Tp>>(std::forward<_Tp>(__value)))
+    { return          ranges::repeat_view<decay_t<_Tp>>(std::forward<_Tp>(__value)); }
 
   template <class _Tp, class _Bound>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Tp&& __value, _Bound&& __bound_sentinel)

libcxx/test/std/ranges/range.factories/range.repeat.view/ctad.compile.pass.cpp

@@ -24,4 +24,10 @@ static_assert(std::same_as<decltype(std::ranges::repeat_view(std::declval<Empty&
 static_assert(std::same_as<decltype(std::ranges::repeat_view(10, 1)), std::ranges::repeat_view<int, int>>);
 static_assert(std::same_as<decltype(std::ranges::repeat_view(10, 1U)), std::ranges::repeat_view<int, unsigned>>);
 static_assert(std::same_as<decltype(std::ranges::repeat_view(10, 1UL)), std::ranges::repeat_view<int, unsigned long>>);
+
+using RPV = std::ranges::repeat_view<const char*>;
+static_assert(std::same_as<decltype(std::ranges::repeat_view("foo", std::unreachable_sentinel)), RPV>);  // OK
+static_assert(std::same_as<decltype(std::ranges::repeat_view(+"foo", std::unreachable_sentinel)), RPV>); // OK
+static_assert(std::same_as<decltype(std::ranges::repeat_view("foo")), RPV>);                             // OK since LWG4053
+static_assert(std::same_as<decltype(std::ranges::repeat_view(+"foo")), RPV>);                            // OK
 // clang-format on

libcxx/test/std/ranges/range.factories/range.repeat.view/views_repeat.pass.cpp

@@ -60,6 +60,18 @@ static_assert(!std::is_invocable_v<decltype(std::views::repeat), NonCopyable>);
 // Tp is move_constructible
 static_assert(std::is_invocable_v<decltype(std::views::repeat), MoveOnly>);
 
+// Test LWG4054 "Repeating a repeat_view should repeat the view"
+static_assert(std::is_same_v<decltype(std::views::repeat(std::views::repeat(42))),
+                             std::ranges::repeat_view<std::ranges::repeat_view<int>>>);
+
+// These cases are from LWG4053, but they are actually covered by the resolution of LWG4054,
+// and the resolution of LWG4053 only affects CTAD.
+using RPV = std::ranges::repeat_view<const char*>;
+static_assert(std::same_as<decltype(std::views::repeat("foo", std::unreachable_sentinel)), RPV>);  // OK
+static_assert(std::same_as<decltype(std::views::repeat(+"foo", std::unreachable_sentinel)), RPV>); // OK
+static_assert(std::same_as<decltype(std::views::repeat("foo")), RPV>);                             // OK since LWG4054
+static_assert(std::same_as<decltype(std::views::repeat(+"foo")), RPV>);                            // OK
+
 constexpr bool test() {
   assert(*std::views::repeat(33).begin() == 33);
   assert(*std::views::repeat(33, 10).begin() == 33);