template parameters

[Christopher Wright]

Ellery Newcomer wrote:
> Okay, let me rephrase that:
> 
> When you have a <something> preceded by a colon in a template value
> parameter, what are its semantics? Is it the same as a default parameter
> value? Is it some sort of constraining expression? Is it a vestige?

void foo(T : int)(T value)
{
	// value is implicitly convertible to int
}

It can do some pattern matching:

void foo(T : V[U], V, U)(T dictionary)
{
	// I have an associative array
}

In D2, it's usually going to be easier to use constraints:

template foo(T) if (isAssociativeArray!T) {}