main.d

import std.stdio;

void main()
{
	HighLevel h;
	writeln(h.fun());
}

struct HighLevel
{
	ComponentA a;
	ComponentB!a b;

	int fun()
	{
		return b.funB();
	}
}

struct ComponentA
{
	int funA()
	{
		return 42;
	}
}

struct ComponentB(alias a)
{
	int funB()
	{
		return a.funA(); // Error: need 'this' for 'a' of type 'ComponentA'
	}
}