Type roles for UArray?

So I was working on porting my libraries to work for GHC 7.10 and I ran into a snag. Unlike the concrete type Data.Array.Array, the concrete type Data.Array.Unboxed.UArray is defined as being nominal in the second type argument— is this a bug? If not, then why must it be nominal? Shouldn't (UArray a New) and (UArray a Old) have the same representation in memory? Using unsafeCoerce has always been safe here in the past... -- Live well, ~wren