UDA initialize a user defined type attribute field

[Arjan]

When a user defined type is used as an attribute like this:
// the attribute
struct MyAttrib
{
     immutable string name;
     imuttable int sz;
     immutable string defaultvalue;
}

// using the attribute
class Data
{
     // field 'id' has attribute MyAttrib with name and sz set to 
'fancy name'
     // and 10 resp.
     @MyAttrib( "fancy name", 10 )
     long id;
     ...
}


But how do I only set the MyAttrib.defaultvalue?

class OtherData
{
     //@MyAttrib( ??defaultvalue = "null" ?? )
     //@MyAttrib( string.init, int.init, "null" ) //seems to work?
     long id;
     ...
}

MyAttrib instances may be initialized like this:

auto myattrib = { defaultvalue:"null" };

But this seems not possible when using MyAttrib as attribute.