cast(A)b is not an lvalue

[Maxim Fomin]

On Wednesday, 26 December 2012 at 17:13:14 UTC, Ali Çehreli wrote:
> Here is the code:
>
> import std.stdio;
>
> static if (!is(typeof(writeln)))
> 	alias writefln writeln;
>

What does this for? I constantly face in code samples shared in 
this NG.