[libcxx-commits] [libcxx] [libc++][mdspan] Fix extents CTAD (PR #68737)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Christian Trott (crtrott)

<details>
<summary>Changes</summary>

extents CTAD was requiring default constructibility of the extent arguments due to the way we implemented a pack expansion. This requirement is not in the standard.

Reported in issue #<!-- -->68671 https://github.com/llvm/llvm-project/issues/68671 by @<!-- -->hewillk.

Fixes #<!-- -->68671

---
Full diff: https://github.com/llvm/llvm-project/pull/68737.diff


2 Files Affected:

- (modified) libcxx/include/__mdspan/extents.h (+1-1) 
- (modified) libcxx/test/std/containers/views/mdspan/extents/ctad.pass.cpp (+7) 


``````````diff
diff --git a/libcxx/include/__mdspan/extents.h b/libcxx/include/__mdspan/extents.h
index a510220d4096a2f..f6bcd940ee6077d 100644
--- a/libcxx/include/__mdspan/extents.h
+++ b/libcxx/include/__mdspan/extents.h
@@ -456,7 +456,7 @@ using dextents = typename __mdspan_detail::__make_dextents<_IndexType, _Rank>::t
 
 // Deduction guide for extents
 template <class... _IndexTypes>
-extents(_IndexTypes...) -> extents<size_t, size_t((_IndexTypes(), dynamic_extent))...>;
+extents(_IndexTypes...) -> extents<size_t, size_t(((void)sizeof(_IndexTypes), dynamic_extent))...>;
 
 namespace __mdspan_detail {
 
diff --git a/libcxx/test/std/containers/views/mdspan/extents/ctad.pass.cpp b/libcxx/test/std/containers/views/mdspan/extents/ctad.pass.cpp
index 2a3da30bb936600..fed16b3fc91b459 100644
--- a/libcxx/test/std/containers/views/mdspan/extents/ctad.pass.cpp
+++ b/libcxx/test/std/containers/views/mdspan/extents/ctad.pass.cpp
@@ -21,6 +21,12 @@
 #include "../ConvertibleToIntegral.h"
 #include "test_macros.h"
 
+struct S {
+  size_t value;
+  constexpr S(size_t val):value(val) {};
+  constexpr operator size_t() const noexcept { return value; }
+};
+
 template <class E, class Expected>
 constexpr void test(E e, Expected expected) {
   ASSERT_SAME_TYPE(E, Expected);
@@ -35,6 +41,7 @@ constexpr bool test() {
   test(std::extents(1, 2u), std::extents<std::size_t, D, D>(1, 2u));
   test(std::extents(1, 2u, 3, 4, 5, 6, 7, 8, 9),
        std::extents<std::size_t, D, D, D, D, D, D, D, D, D>(1, 2u, 3, 4, 5, 6, 7, 8, 9));
+  test(std::extents(S{1}, S{2}), std::extents<std::size_t, D, D>(1, 2u));
   return true;
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/68737