Template type reduction

[Lodovico Giaretta via Digitalmars-d-learn]

On Monday, 15 August 2016 at 19:31:14 UTC, 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 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!*.
>
>
> Is this possible?

I don't know if this is exactly what you want:

=====================================
import std.traits: hasMember;

struct Q(T)
     if (hasMember!(T, "x"))
{
     T s;

     @property auto x() { return s.x; }
}
auto makeQ(T)(auto ref T val)
{
     return Q!T(val);
}

auto s = myTypeWithFieldX();
auto q = makeQ(s);
assert(q.x == s.x);
=====================================

`Q` can store any type with an `x` field and gives access to it. 
The auxiliary function `makeQ` acts as a constructor for `Q` with 
template parameter deduction.