Subtypes with tighter constraints

[Neia Neutuladh]

On Tue, 01 Jan 2019 14:05:43 +0000, 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?

In D, structs don't participate in inheritance. Classes do, but invariants 
aren't inherited; they're specific to the class in which the function is 
defined. (Which is probably a bug and I filed https://issues.dlang.org/
show_bug.cgi?id=19537.)

I see three ways to make this work today:

1. Use alias this to wrap the struct. Add invariants that deal with the 
wrapped struct's fields.
2. Use compile-time reflection to copy the fields over, then manually copy 
the invariants over and add more.
3. Use a class. Define a virtual function like 'doInvariant()`. Call it 
from the base class's invariant{} block.

> As I understand if I derive Y from X, then it is no more X; that is I
> cannot use X (even provided it matches Y invariants) where I need Y. So
> in D it is impossible, right?

With classes, a derived class instance can be used anywhere you expect a 
base class instance. The reverse is not true.