How to check whether a struct is templated?

[Balagopal Komarath via Digitalmars-d-learn]

Are you looking for something like this?

import std.typecons;
import std.traits;

alias yes = Nullable!int;

struct no {}

template isNullable(T : Nullable!X, X)
{
     enum isNullable = true;
}

template isNullable(T)
{
     enum isNullable = false;
}

void main()
{
     static assert(isNullable!yes);
     static assert(!isNullable!no);
}