Deducing a template argument from an aliased parameter

[anonymous via Digitalmars-d-learn]

On 31.12.2015 23:37, rcorre wrote:
> struct Vector(T, int N) { }
> alias Vector2(T) = Vector!(T, 2);
>
> void fun1(T)(Vector!(T, 2) vec) { }
> void fun2(T)(Vector2!T vec) { }
>
> unittest {
>    fun1(Vector!(float, 2).init);
>    fun2(Vector!(float, 2).init);
> }
>
> Why can fun1 deduce `T`, but fun2 can't?
>
> Failure:
> "template d.fun2 cannot deduce function from argument types
> !()(Vector!(float, 2))"?

Vector2 is a little more than just an alias, it's a template for aliases.

Your Vector2 is short for this:
----
template Vector2(T)
{
     alias Vector2 = Vector!(T, 2);
}
----

You can see that such a template could map different T types to the same 
result type. For example, Vector2!int and Vector2!long could both become 
aliases to Vector!(long, 2). Deducing T from a Vector!(long, 2) argument 
would be ambiguous then. T could be int or long, and there is no way to 
tell what it should be.

That's just how I make sense of this, though. I'm not sure if it's the 
whole picture.