Fix span for non-ranges

We did not properly implement the short circuiting for the `__span_compatible_range` concept. So the compiler tried to instantiate `iterator_t` even for non-ranges, which obviously failed.

Fix this by properly implementing short circuiting.

libcudacxx/include/cuda/std/detail/libcxx/include/span

@@ -204,12 +204,19 @@ _LIBCUDACXX_CONCEPT __span_array_convertible = _CCCL_TRAIT(is_convertible, _From
 
 #  if _CCCL_STD_VER >= 2017 && !defined(_CCCL_COMPILER_MSVC_2017)
 template <class _Range, class _ElementType>
-_LIBCUDACXX_CONCEPT __span_compatible_range =
-  _CUDA_VRANGES::contiguous_range<_Range> && _CUDA_VRANGES::sized_range<_Range>
-  && (_CUDA_VRANGES::borrowed_range<_Range> || _CCCL_TRAIT(is_const, _ElementType))
-  && !__is_std_span<remove_cvref_t<_Range>> && !__is_std_array<remove_cvref_t<_Range>>
-  && !_CCCL_TRAIT(is_array, remove_cvref_t<_Range>)
-  && _CCCL_TRAIT(is_convertible, remove_reference_t<_CUDA_VRANGES::range_reference_t<_Range>> (*)[], _ElementType (*)[]);
+_LIBCUDACXX_CONCEPT_FRAGMENT(
+  __span_compatible_range_,
+  requires()(
+    requires(_CUDA_VRANGES::contiguous_range<_Range>),
+    requires(_CUDA_VRANGES::sized_range<_Range>),
+    requires((_CUDA_VRANGES::borrowed_range<_Range> || _CCCL_TRAIT(is_const, _ElementType))),
+    requires((!_CCCL_TRAIT(is_array, remove_cvref_t<_Range>))),
+    requires((!__is_std_span<remove_cvref_t<_Range>> && !__is_std_array<remove_cvref_t<_Range>>) ),
+    requires(_CCCL_TRAIT(
+      is_convertible, remove_reference_t<_CUDA_VRANGES::range_reference_t<_Range>> (*)[], _ElementType (*)[]))));
+
+template <class _Range, class _ElementType>
+_LIBCUDACXX_CONCEPT __span_compatible_range = _LIBCUDACXX_FRAGMENT(__span_compatible_range_, _Range, _ElementType);
 
 #    if _CCCL_STD_VER >= 2020
 template <class _It, class _Tp>

libcudacxx/test/libcudacxx/std/containers/views/views.span/span.cons/conversion.pass.cpp

@@ -0,0 +1,47 @@
+//===---------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===---------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11
+
+// <cuda/std/span>
+
+// Conversion from a type that is *not* a range. We had a bug where we would still try to instantiate `iterator_t`,
+// which would fail because of a missing `begin`
+
+#include <cuda/std/cassert>
+#include <cuda/std/span>
+
+#include "test_macros.h"
+
+struct ConvertibleButNoRange
+{
+  int buffer[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+
+  __host__ __device__ constexpr operator cuda::std::span<int>() const noexcept
+  {
+    return cuda::std::span<int>{const_cast<int*>(buffer), 10};
+  }
+};
+
+__host__ __device__ constexpr bool test()
+{
+  ConvertibleButNoRange input{};
+  cuda::std::span<int> converted = input;
+  assert(converted.data() == input.buffer);
+  assert(converted.size() == 10);
+
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+  static_assert(test(), "");
+
+  return 0;
+}