[libcxx-commits] [libcxx] [libc++] Fix the mdspan ElementType complete object type mandate (PR #191703)

[via libcxx-commits]

https://github.com/eiytoq updated https://github.com/llvm/llvm-project/pull/191703

>From 60889578fdb7a31740e821b4b6e0ab7d220a486d Mon Sep 17 00:00:00 2001
From: eiytoq <eiytoq at outlook.com>
Date: Sun, 12 Apr 2026 20:25:28 +0800
Subject: [PATCH 1/2] fix

---
 libcxx/include/__mdspan/mdspan.h                    |  4 ++++
 .../views/mdspan/mdspan/element_type.verify.cpp     | 13 +++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index 449baea43f2d7..b8f24b9ac0c31 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -30,6 +30,7 @@
 #include <__type_traits/is_constructible.h>
 #include <__type_traits/is_convertible.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_object.h>
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/rank.h>
@@ -66,6 +67,9 @@ class mdspan {
 private:
   static_assert(__mdspan_detail::__is_extents_v<_Extents>,
                 "mdspan: Extents template parameter must be a specialization of extents.");
+  static_assert(
+      is_object_v<_ElementType> && requires { sizeof(_ElementType); },
+      "mdspan: ElementType template parameter must be a complete object type");
   static_assert(!is_array_v<_ElementType>, "mdspan: ElementType template parameter may not be an array type");
   static_assert(!is_abstract_v<_ElementType>, "mdspan: ElementType template parameter may not be an abstract class");
   static_assert(is_same_v<_ElementType, typename _AccessorPolicy::element_type>,
diff --git a/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp b/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp
index 9f2730262d953..8196788b1fe77 100644
--- a/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp
+++ b/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp
@@ -23,6 +23,19 @@ class AbstractClass {
   virtual void method() = 0;
 };
 
+struct BadAccessor {
+  using offset_policy    = BadAccessor;
+  using element_type     = void;
+  using reference        = void;
+  using data_handle_type = element_type*;
+  reference access(data_handle_type, std::size_t) const;
+};
+
+int incomplete_type() {
+  // expected-error-re@*:* {{static assertion failed {{.*}}mdspan: ElementType template parameter must be a complete object type}}
+  [[maybe_unused]] std::mdspan<void, std::dextents<std::size_t, 2>, std::layout_right, BadAccessor> m;
+}
+
 void not_abstract_class() {
   // expected-error-re@*:* {{static assertion failed {{.*}}mdspan: ElementType template parameter may not be an abstract class}}
   [[maybe_unused]] std::mdspan<AbstractClass, std::extents<int>> m;

>From 4dbc5033cfefeea9c65cd3fef058892a43d78e0a Mon Sep 17 00:00:00 2001
From: eiytoq <eiytoq at outlook.com>
Date: Sun, 12 Apr 2026 22:48:12 +0800
Subject: [PATCH 2/2] Addressed comment

Co-authored-by: A. Jiang <de34 at live.cn>
---
 .../std/containers/views/mdspan/mdspan/element_type.verify.cpp  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp b/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp
index 8196788b1fe77..7a6bde5b76367 100644
--- a/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp
+++ b/libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.cpp
@@ -31,7 +31,7 @@ struct BadAccessor {
   reference access(data_handle_type, std::size_t) const;
 };
 
-int incomplete_type() {
+void incomplete_type() {
   // expected-error-re@*:* {{static assertion failed {{.*}}mdspan: ElementType template parameter must be a complete object type}}
   [[maybe_unused]] std::mdspan<void, std::dextents<std::size_t, 2>, std::layout_right, BadAccessor> m;
 }