Cannot cast X to Y at compile time...?

[Frustrated]

On Wednesday, 19 March 2014 at 16:57:38 UTC, dnspies wrote:
> I have a function called at CTFE which includes the lines:
>
> 97	if(conjunction exp = cast(conjunction)this_exp) {
> 98		inner_substitutions!(first,conjunction)(exp, map);
> 99	} else if(disjunction exp = cast(disjunction)this_exp) {
> 100		inner_substitutions!(first,disjunction)(exp, map);
> 101	}
>
> Here, this_exp is a reference of type "expression" to an object 
> whose CTFE-runtime-type is of type "disjunction".  conjunction 
> and disjunction are both descendent classes of expression.
> This code produces the following compilation error:
>
> source/cfgparse.d(97): Error: cannot cast [...] to 
> cfgparse.conjunction at compile time.
>
> ([...] stands in for a very long string which I think is some 
> sort of representation of this_exp)
>
> Just for the hell of it, I tried moving the assignment out of 
> the conditional, and something very strange happens.
>
> 97	if(cast(conjunction)this_exp) {
> 98		conjunction exp = cast(conjunction)this_exp;
> 99		inner_substitutions!(first,conjunction)(exp, map);
> 100	} else if(cast(disjunction)this_exp) {
> 101		disjunction exp = cast(disjunction)this_exp;
> 102		inner_substitutions!(first,disjunction)(exp, map);
> 103	}
>
> source/cfgparse.d(101): Error: cannot cast [...] to 
> cfgparse.disjunction at compile time
>
> Both the conditions compile properly, and now only the 
> assignment fails.  Why  is this happening and how can I avoid 
> it?

I ran up to a similar situation when the thing trying to be cast 
was not what I thought it was. I.e., the error is exactly what it 
means. Try to create a this_exp that is a conjunction explicitly 
to see if that is the problem.