Detecting if a class type (which may or may not have a default constructor) is abstract

[Christopher Wright]

Jarrett Billingsley wrote:
> I just don't think it's possible.  If all classes had default ctors,
> it'd be easy; is(typeof(new T)) would be false if and only if T were
> abstract.  But since that's not the case, I can't think of a way to
> generically see if a given class type is abstract.  Any ideas?
> 
> It's always a little frustrating when doing type introspection and
> having to rely on weird side-effects and properties of types, when the
> compiler is just keeping it in some flag or field somewhere.  Sigh.
> "is(T == abstract)"?  :P

If you know the constructor arguments in advance, you can do something like:
static if (is (typeof (new Foo (1, "hello")))){}

Unfortunately, ParameterTupleOf!(T._ctor) doesn't work:

class AFoo {}
if (is (typeof (AFoo._ctor))) Stdout.formatln ("AFoo._ctor");
if (is (typeof (ParameterTupleOf!(AFoo._ctor)))) Stdout.formatln 
("AFoo._ctor params");
// prints AFoo._ctor

_ctor is a really odd construct -- spotty support, not advertised.