Template type reduction

[Engine Machine via Digitalmars-d-learn]

On Monday, 15 August 2016 at 19:40:37 UTC, Steven Schveighoffer 
wrote:
> On 8/15/16 3:31 PM, Engine Machine wrote:
>> Suppose I have a templated type like
>>
>> struct S(T) { int x; static if (T is Y) int y; }
>>
>> I would like to be able to create a reference to S(T) for any 
>> T,
>>
>> struct Q
>> {
>>   S!* s; // Can hold any type of S.
>> }
>>
>> and be able to access s.x, since it is common to all S.
>>
>> Can D do anything like this? It is sort of like runtime 
>> inheritance, but
>> at the compile time level.
>
> I don't think so. You'd have to cast, as the compiler doesn't 
> have any understanding that all S instantiations will have an x 
> member.

This is a problem. How can I cast to something I don't know?

>
>> I do not want to have to cast to S!T every time just to access 
>> x, e.g.,
>>
>> struct Q
>> {
>>    Object s;
>> }
>>
>> which is too general as s can be things that are not of type 
>> S!*.
>
> This seems odd. You will lose the type information for s if you 
> were to succeed. Might as well just store an int.

> Seems like what you want is a variant or some other kind of 
> tagged union.

No, what I want is a type that is the intersection of all the 
types it can be.

The problem is I cannot cast because the type is complex and I 
don't know the exact signature, but I do know that it has a basic 
type embedded in it.

enum Types
{
    Int, Word, Complex
}

struct MyType(string name, Types type, Args...)
{
     string Name = name;
     Types Type = type;
     static if (type == Int) int val;
     static if (type == word) ushort val;
     static if (type == Complex) { ... };
}


So, how can I get val? I know if it is an int or a word, val 
exists. But I can't specify the name or type at runtime to get at 
the innards of the type.

Object o = MyType!("test", Types.Int);

Useless! I can never retrieve anything from o. I can't cast it 
back. Yet I know that o has a name and a type and possibly a val 
if I know the type... and I know all this at compile time. Yet D 
doesn't seem to allow me to use this information.

I know one will say use either oop or constructors, but neither 
of these are appropriate(although constructors are close). What I 
need is a type constructor, which bridges the gap. This is how I 
am using the data above, to construct the object, but D treats it 
as part of the type, which I don't what.

I guess I will have to use string mixins to construct the types 
properly ;/ What a pain in the ass ;/