Suggestion for improving the sycl_ext_oneapi_local_memory extension

[fabiomestre]

The sycl_ext_oneapi_local_memory extension returns a sycl::multi_ptr
to point to the local memory. However, the syntax to access the local memory can be confusing.

The main issue is that multi_ptr also implements the [] operator. This can lead
to subtle errors. For example:

multi_ptr<size_t[LocalSize], access::address_space::local_space>
    LocalMemPtr = sycl::ext::oneapi::group_local_memory<size_t[LocalSize]>(
        Item.get_group());

// Incorrect: Uses the operator overload of multi_ptr.
*LocalMemPtr[Item.get_local_linear_id()] = Item.get_global_linear_id();

// Correct: Dereferences pointer first.
 (*LocalMemPtr)[Item.get_local_linear_id()] = Item.get_global_linear_id();

I'm wondering if, either this extension or multi_ptr, could be improved to make these kinds of user errors detectable at compile time? For example, when the type is a C array, it doesn't make sense to use the [] operator of multi_ptr. Maybe a static_assert could be added or the operator could be disabled in that case?