Unlike generator the allocator customisation of task constraints the allocator type used for the coroutine to be convertible to the configured allocator_type. This prevents easy use of an allocator especially when no allocator is configured and the default (std::allocator<std::byte>) is used. The reason for this constraint is that the get_allocator is forwarded to co_awaited senders and is intended to be the same as the allocator used for the coroutine frame.

It may be reasonable to allow use of an arbitrary allocator when there is no explicit configuration of the allocator_type. In this case it may also be resonable to not support the get_allocator query when co_awaiting senders.