why ushort alias casted to int?

[crimaniak via Digitalmars-d-learn]

My code:

alias MemSize = ushort;
struct MemRegion
{
        MemSize start;
        MemSize length;
        @property MemSize end() const { return start+length; }
}

Error: cannot implicitly convert expression `cast(int)this.start 
+ cast(int)this.length` of type `int` to `ushort`

Both operands are the same type, so as I understand casting to 
longest type is not needed at all, and longest type here is 
ushort in any case. What am I doing wrong?