Difficulties with std.variant.Algebraic

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On Thursday, 11 July 2013 at 18:31:50 UTC, Jesse Phillips wrote:
> I forgot to mention that most of the casting you were doing 
> doesn't do what you think it would.
> ...
>
> This doesn't make an Option!int castable to a Maybe!int, they 
> are still distinct types:
> ...

struct Test1
{
	int n;
}

struct Test2
{
	string s;
	Test1 t;
	
	alias t this;
}

void main()
{
	auto t1 = Test1();
	auto t2 = cast(Test2)t1; //Error
}

It seems you're right. That's disappointing. TDPL talks about 
alias this in terms of subtyping, but doesn't the above code show 
that this is not true subtyping?