How use "toHash" without cast

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> It's actually @trusted as per the docs. I do not know why DMD 
> infers that to be @safe.....
>
> In any case, the solution here is this:
>
>     override hash_t toHash() @trusted const pure nothrow {
>         return cast(hash_t)(this.x + this.y);
>     }

Yes, it's already trusted.

If I write "@trusted const pure nothrow" it works fine, but if i 
write "const pure nothrow" not. Why? I thought the compiler is 
orientating at the base method which is overriden.