Subtypes with tighter constraints

[Alex]

On Tuesday, 1 January 2019 at 14:05:43 UTC, Victor Porton wrote:
> In Ada2012 there are "subtypes". Subtypes can have tighter 
> constraints (such as type invariants) than their base types.
>
> I have a struct X in D. Is it possible to define a type 
> equivalent to X except that having tighter invariants?
>
> As I understand if I derive Y from X, then it is no more X;
1. As Ada is strongly typed, this is the same there, no?
2. You cannot derive structs in D. Therefore, I assume, that you 
meant that X and Y are classes.

> that is I cannot use X (even provided it matches Y invariants) 
> where I need Y. So in D it is impossible, right?

It is possible, as invariants are inherited implicitly, see
https://dlang.org/spec/contracts.html#Invariants
paragraph 9.

´´´
import std.experimental.all;

void main()
{
	auto x = new X();
	auto y = new Y();
	x.d = 0.0;
	y.d = 1.0;
	x.fun;
	y.fun;
}

class X
{
	double d;

	invariant
	{
		assert(!d.isNaN);
	}
}

class Y : X
{
	invariant
	{
		assert(d > 0);
	}
}

void fun(T)(T t)
{
	assert(t);
}
´´´