What is this behavior and how do I disable or get around it?

[pineapple via Digitalmars-d-learn]

This program does not compile.

Error: cannot implicitly convert expression (cast(int)x - 
cast(int)x) of type int to ubyte

    void main(){
        ubyte x;
        x = x - x;
    }

I don't even know what to say. Who thought this behavior was a 
good idea?