[libcxx-commits] [libcxx] [libc++][mdspan] P3383R3: `mdspan.at()` (PR #175213)

[A. Jiang via libcxx-commits]

================
@@ -217,6 +219,37 @@ class mdspan {
     }(make_index_sequence<rank()>()));
   }
 
+#  if _LIBCPP_STD_VER >= 26
+  template <class... _OtherIndexTypes>
+    requires((is_convertible_v<_OtherIndexTypes, index_type> && ...) &&
+             (is_nothrow_constructible_v<index_type, _OtherIndexTypes> && ...) &&
+             (sizeof...(_OtherIndexTypes) == rank()))
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference at(_OtherIndexTypes... __indices) const {
+    if (!__mdspan_detail::__is_multidimensional_index_in(__map_.extents(), __indices...)) {
+      __throw_out_of_range();
+    }
+    return (*this)[extents_type::__index_cast(std::move(__indices))...];
+  }
+
+  template <class _OtherIndexType>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference at(const array<_OtherIndexType, rank()>& __indices) const {
+    return [&]<size_t... _Idxs>(index_sequence<_Idxs...>) -> reference {
+      return at(extents_type::__index_cast(as_const(__indices[_Idxs]))...);
----------------
frederick-vs-ja wrote:

`__indices[_Idxs]...` are obviously already `const`.
```suggestion
      return at(extents_type::__index_cast(__indices[_Idxs])...);
```

https://github.com/llvm/llvm-project/pull/175213