https://godbolt.org/z/f4dc83fr3

std::vector v{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
auto ms2 = std::mdspan(v.data(), 2, 6);
ms2[0, 0] = 3;

error C2676: binary ‘[‘: ‘std::mdspan<_Ty,std::extents<_IndexType,18446744073709551615,18446744073709551615>,std::layout_right,std::default_accessor<_Ty>>’ does not define this operator or a conversion to a type acceptable to the predefined operator