Why is value of `_tokenApprovals` mapping a struct with one parameter instead simply an address?

[Lruquaf]

struct TokenApprovalRef {
        address value;
}

// Mapping from token ID to approved address.
    mapping(uint256 => TokenApprovalRef) private _tokenApprovals;

[Vectorized]

This is a workaround for a old compiler bug.

[Lruquaf]

Thanks!