Bad error message when deriving(Clone) is missing Clone for a member

[kmcallister]

Compiling

#[deriving(Clone)]
enum ImageState {
    Init,
    Prefetching(AfterPrefetch),
    Prefetched(@Cell<~[u8]>),
    Decoding,
    Decoded(@Arc<~Image>),
    Failed
}

enum AfterPrefetch {
    DoDecode,
    DoNotDecode
}

produces

error: mismatched types: expected `image_cache_task::AfterPrefetch` but found `&image_cache_task::AfterPrefetch` (expected enum image_cache_task::AfterPrefetch but found &-ptr)
#[deriving(Clone)]
           ^~~~~

The solution is just to derive Clone on AfterPrefetch as well. But the error message is not helpful.

[alexcrichton]

Thanks for the report! Closing as a dupe of #7724, however.