Hello! Thanks for the notice. I've been enjoying delving into D recently, and have made quite some progress, but I've become stumped on this one problem!
I consider myself decent at natural debugging, but this problem has eluded me.

I don't believe any of this problem is implementation specific to the rest of my project, but please note if this is too vague. I'm defining something like..

class Woah(){}
class Bro: Woah{}
DList!Woah woahs;

and I'm having trouble with..

foreach( bro; woahs.filter!( a => cast(Bro)a !is null))

I'd figure that this would enumerate a collection of Woahs that are in fact Bros. Maybe I'm just spoiled by Linq.
Instead, I'm getting hit by this.

Error: template std.algorithm.filter cannot deduce function from argument types !()(DList!(Woah), void), candidates are:
..\src\phobos\std\algorithm.d(1628):
std.algorithm.filter(alias pred) if (is(typeof(unaryFun!pred)))

[ Likewise if I specify by filter!( func)( collection) ]

It seems to me that maybe it's a problem with the predicate I'm supplying; even though it's unary.
Any help, or how I can proceed and remove my eyesore placeholder will be greatly appreciated.
Thanks!

meat:

> class Woah(){}
> class Bro: Woah{}
> DList!Woah woahs;
>
> and I'm having trouble with..
>
> foreach( bro; woahs.filter!( a => cast(Bro)a !is null))


import std.algorithm, std.container;

class Woah {}
class Bro : Woah {}

void main() {
    DList!Woah woahs;
    foreach (bro; woahs[].filter!(a => cast(Bro)a !is null)) {}
}

Bye,
bearophile