unittest {
    Mass m;
    Velocity v;
    pragma(msg, typeof(m * v).stringof);
    pragma(msg, typeof(v * m).stringof);
    static assert(is(typeof(m * v) == double));
    static assert(is(typeof(v * m) == double));
}

Currently the type of m * v is Velocity, and the type of v * m is Mass. The only reasonable type of an operation (if it compiles at all) should be double, obtained after both operands are converted to their supertype.