----
import std.stdio;

struct Foo(T = int) { }

void main()
{
	Foo f;
}
----
Error: Error: struct Foo(T = int) is used as a type

If I change Foo f into Foo!() f it works. Is the compiler not able to see that this is not necessary, since I have a default type?

I wondered the same thing a couple of weeks back. It seems to
work this way to avoid certain ambiguities. Related bug report /
discussion:

http://d.puremagic.com/issues/show_bug.cgi?id=1012