From f13ff250a89da6bae7e336f74c8f3f19aecde1fb Mon Sep 17 00:00:00 2001
From: Michael Schellenberger Costa <miscco@nvidia.com>
Date: Fri, 5 Jun 2026 09:17:49 +0200
Subject: [PATCH] [libcu++] Fix `__is_sequence` definition for arguments
 framework

The  current is_sequence definition is thoroughly broken. An iterator is not a sequence and neither is a pointer.

The main problem here is that this precludes passing along iterators / pointers as values which is a reasonable use case.

It also completely deviates from any standard handling in C++.

Rather than reinventing the wheel in a bad way use standard terminology to determine what is a range and what not.

A user can always wrap iterators and pointers into views and spans, so there is not need for the current definition
---
 libcudacxx/include/cuda/__argument/argument.h | 23 +++-------
 .../cuda/argument/argument_traits.pass.cpp    | 45 ++++++++++---------
 .../cuda/argument/dynamic_argument.pass.cpp   |  2 +-
 .../cuda/argument/static_argument.pass.cpp    |  2 +-
 4 files changed, 34 insertions(+), 38 deletions(-)

diff --git a/libcudacxx/include/cuda/__argument/argument.h b/libcudacxx/include/cuda/__argument/argument.h
index fb514d22648..697b580ca53 100644
--- a/libcudacxx/include/cuda/__argument/argument.h
+++ b/libcudacxx/include/cuda/__argument/argument.h
@@ -26,7 +26,9 @@
 #include <cuda/std/__algorithm/min_element.h>
 #include <cuda/std/__cccl/assert.h>
 #include <cuda/std/__iterator/readable_traits.h>
+#include <cuda/std/__ranges/concepts.h>
 #include <cuda/std/__type_traits/is_arithmetic.h>
+#include <cuda/std/__type_traits/is_array.h>
 #include <cuda/std/__type_traits/is_integer.h>
 #include <cuda/std/__type_traits/is_integral.h>
 #include <cuda/std/__type_traits/is_same.h>
@@ -75,23 +77,12 @@ template <class _Tp>
 using __element_type_of_t = typename __element_type_of<::cuda::std::remove_cvref_t<_Tp>>::type;
 
 // =====================================================================
-// __is_sequence_v / __is_single_value_v
+// __is_sequence_v
 // =====================================================================
 
 template <class _Tp>
 inline constexpr bool __is_sequence_v =
-  !::cuda::std::is_same_v<::cuda::std::remove_cvref_t<_Tp>, __element_type_of_t<_Tp>>;
-
-template <class _Tp>
-inline constexpr bool __is_single_value_v = !__is_sequence_v<_Tp>;
-
-template <class _Tp, class = void>
-inline constexpr bool __is_iterable_v = false;
-
-template <class _Tp>
-inline constexpr bool __is_iterable_v<_Tp,
-                                      ::cuda::std::void_t<decltype(::cuda::std::declval<const _Tp&>().begin()),
-                                                          decltype(::cuda::std::declval<const _Tp&>().end())>> = true;
+  ::cuda::std::is_array_v<::cuda::std::remove_cvref_t<_Tp>> || ::cuda::std::ranges::range<_Tp>;
 
 // =====================================================================
 // __constant
@@ -121,7 +112,7 @@ struct __constant_sequence
   using value_type     = ::cuda::std::remove_cvref_t<decltype(_Value)>;
   using __element_type = __element_type_of_t<value_type>;
 
-  static_assert(__is_sequence_v<value_type>, "constant sequence arguments must have a distinct element type");
+  static_assert(__is_sequence_v<value_type>, "The value type of __constant_sequence must be a range or an array");
 
   [[nodiscard]] _CCCL_API static constexpr value_type value() noexcept
   {
@@ -332,7 +323,7 @@ struct __immediate_sequence
 
   _CCCL_API constexpr void __validate_value() const noexcept
   {
-    if constexpr (__is_iterable_v<_Arg> && ::cuda::std::is_arithmetic_v<__element_type>)
+    if constexpr (__is_sequence_v<_Arg> && ::cuda::std::is_arithmetic_v<__element_type>)
     {
       for (const auto& __a : __arg_)
       {
@@ -671,7 +662,7 @@ struct __traits_impl
   using element_type                    = __element_type_of_t<_Tp>;
   static constexpr bool is_constant     = false;
   static constexpr bool is_deferred     = false;
-  static constexpr bool is_single_value = __is_single_value_v<_Tp>;
+  static constexpr bool is_single_value = !__is_sequence_v<_Tp>;
   static constexpr element_type lowest  = ::cuda::std::numeric_limits<element_type>::lowest();
   static constexpr element_type highest = (::cuda::std::numeric_limits<element_type>::max)();
 };
diff --git a/libcudacxx/test/libcudacxx/cuda/argument/argument_traits.pass.cpp b/libcudacxx/test/libcudacxx/cuda/argument/argument_traits.pass.cpp
index cdcc6665747..36154ef2207 100644
--- a/libcudacxx/test/libcudacxx/cuda/argument/argument_traits.pass.cpp
+++ b/libcudacxx/test/libcudacxx/cuda/argument/argument_traits.pass.cpp
@@ -11,10 +11,15 @@
 #include <cuda/__argument_>
 #include <cuda/iterator>
 #include <cuda/std/array>
+#include <cuda/std/complex>
+#include <cuda/std/expected>
 #include <cuda/std/limits>
 #include <cuda/std/mdspan>
+#include <cuda/std/optional>
 #include <cuda/std/span>
+#include <cuda/std/tuple>
 #include <cuda/std/type_traits>
+#include <cuda/std/utility>
 
 #include "test_macros.h"
 
@@ -50,32 +55,32 @@ TEST_FUNC void test()
 {
   // --- __is_sequence_v / __is_single_value_v ---
 
+  // builtin and class type are not sequences
   static_assert(!cuda::__argument::__is_sequence_v<int>);
   static_assert(!cuda::__argument::__is_sequence_v<color>);
   static_assert(!cuda::__argument::__is_sequence_v<non_sequence_value>);
-  static_assert(cuda::__argument::__is_sequence_v<int*>);
-  static_assert(cuda::__argument::__is_sequence_v<cuda::counting_iterator<int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<range_like<int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<element_type_like<int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<value_type_like<int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<cuda::std::complex<float>>);
+  static_assert(!cuda::__argument::__is_sequence_v<cuda::std::pair<float, int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<cuda::std::tuple<float, int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<cuda::std::optional<int>>);
+  static_assert(!cuda::__argument::__is_sequence_v<cuda::std::expected<int, int>>);
+
+  // iterators are not sequences and need to be wrapped into a range
+  static_assert(!cuda::__argument::__is_sequence_v<int*>);
+  static_assert(!cuda::__argument::__is_sequence_v<cuda::counting_iterator<int>>);
+
+  // ranges and arrays are sequences
+  static_assert(cuda::__argument::__is_sequence_v<int[]>);
+  static_assert(cuda::__argument::__is_sequence_v<const int[]>);
+  static_assert(cuda::__argument::__is_sequence_v<int[42]>);
+  static_assert(cuda::__argument::__is_sequence_v<const int[42]>);
   static_assert(cuda::__argument::__is_sequence_v<cuda::std::span<int, 1>>);
   static_assert(cuda::__argument::__is_sequence_v<const cuda::std::span<int, 1>&>);
   static_assert(cuda::__argument::__is_sequence_v<cuda::std::span<int>>);
   static_assert(cuda::__argument::__is_sequence_v<cuda::std::array<int, 3>>);
-  static_assert(cuda::__argument::__is_sequence_v<range_like<int>>);
-  static_assert(cuda::__argument::__is_sequence_v<element_type_like<int>>);
-  static_assert(cuda::__argument::__is_sequence_v<value_type_like<int>>);
-
-  static_assert(cuda::__argument::__is_single_value_v<int>);
-  static_assert(cuda::__argument::__is_single_value_v<float>);
-  static_assert(cuda::__argument::__is_single_value_v<double>);
-  static_assert(cuda::__argument::__is_single_value_v<const int>);
-  static_assert(cuda::__argument::__is_single_value_v<color>);
-  static_assert(cuda::__argument::__is_single_value_v<non_sequence_value>);
-  static_assert(!cuda::__argument::__is_single_value_v<int*>);
-  static_assert(!cuda::__argument::__is_single_value_v<cuda::counting_iterator<int>>);
-  static_assert(!cuda::__argument::__is_single_value_v<cuda::std::span<int, 1>>);
-  static_assert(!cuda::__argument::__is_single_value_v<const cuda::std::span<int, 1>&>);
-  static_assert(!cuda::__argument::__is_single_value_v<cuda::std::span<int>>);
-  static_assert(!cuda::__argument::__is_single_value_v<cuda::std::span<int, 4>>);
-  static_assert(!cuda::__argument::__is_single_value_v<cuda::std::array<int, 3>>);
 
   // --- __element_type_of_t ---
 
@@ -106,7 +111,7 @@ TEST_FUNC void test()
   // --- argument_traits: is_single_value ---
 
   static_assert(cuda::__argument::__traits<int>::is_single_value);
-  static_assert(!cuda::__argument::__traits<int*>::is_single_value);
+  static_assert(cuda::__argument::__traits<int*>::is_single_value);
   static_assert(cuda::__argument::__traits<cuda::__argument::__immediate<int>>::is_single_value);
   static_assert(cuda::__argument::__traits<cuda::__argument::__immediate<int*>>::is_single_value);
   static_assert(
diff --git a/libcudacxx/test/libcudacxx/cuda/argument/dynamic_argument.pass.cpp b/libcudacxx/test/libcudacxx/cuda/argument/dynamic_argument.pass.cpp
index eac5e71aa82..c9723304774 100644
--- a/libcudacxx/test/libcudacxx/cuda/argument/dynamic_argument.pass.cpp
+++ b/libcudacxx/test/libcudacxx/cuda/argument/dynamic_argument.pass.cpp
@@ -128,7 +128,7 @@ TEST_FUNC constexpr bool test()
   // __is_single_value_v on unwrapped types
   {
     static_assert(
-      cuda::__argument::__is_single_value_v<cuda::__argument::__traits<cuda::__argument::__immediate<int>>::value_type>);
+      !cuda::__argument::__is_sequence_v<cuda::__argument::__traits<cuda::__argument::__immediate<int>>::value_type>);
     static_assert(
       !cuda::__argument::__traits<cuda::__argument::__immediate_sequence<cuda::std::span<int>>>::is_single_value);
   }
diff --git a/libcudacxx/test/libcudacxx/cuda/argument/static_argument.pass.cpp b/libcudacxx/test/libcudacxx/cuda/argument/static_argument.pass.cpp
index 7d40183070b..85ccaf1c8a0 100644
--- a/libcudacxx/test/libcudacxx/cuda/argument/static_argument.pass.cpp
+++ b/libcudacxx/test/libcudacxx/cuda/argument/static_argument.pass.cpp
@@ -113,7 +113,7 @@ TEST_FUNC void test()
   // Single value: scalar is single, sequence is not
   {
     static_assert(
-      cuda::__argument::__is_single_value_v<cuda::__argument::__traits<cuda::__argument::__constant<42>>::value_type>);
+      !cuda::__argument::__is_sequence_v<cuda::__argument::__traits<cuda::__argument::__constant<42>>::value_type>);
 #if TEST_HAS_CLASS_NTTP
     static_assert(!cuda::__argument::__traits<
                   cuda::__argument::__constant_sequence<cuda::std::array<int, 3>{1, 2, 3}>>::is_single_value);