Why can't templates with default arguments be instantiated without the bang syntax?

[Steven Schveighoffer]

On Thu, 15 Sep 2011 10:46:24 -0400, Andrej Mitrovic  
<andrej.mitrovich at gmail.com> wrote:

> struct Foo(T = int) {}
>
> void main()
> {
>     Foo foo;  // fail
>     Foo!() bar;  // ok
> }
>
> It would be very convenient to be able to default to one type like this.
>
> For example, in CairoD there's a Point structure which takes doubles
> as its storage type, and then there's PointInt that takes ints. The
> reason they're not both a template Point() that takes a type argument
> is because in most cases the user will use the Point structure with
> doubles, and only in rare cases Point with ints. So to simplify code
> one doesn't have to write Point!double in all of their code, but
> simply Point.
>
> If the bang syntax wasn't required in presence of default arguments
> then these workarounds wouldn't be needed.

Perhaps a different approach:

struct PointT(T) {...}

alias PointT!(double) Point;

// and if so desired:

alias PointT!int PointInt;

Just a thought...

-Steve