Defining a version after it's tested for

[Robert Clipsham]

I recently came across this:

version( BackendFoo ) {}
else version( BackendBar ) {}
else version( BackendCar ) {}
else
{
   pragma( msg, "Warning: backend version undefined" );
   pragma( msg, "Attempting to guess backend" );
   version( Windows )
   {
     version = BackendFoo;
     pragma( msg, "Selected the Foo backend." );
   }
   else version(...)
   { /* You get the idea */ }
}

However, when trying to compile I get the following error:

Error: version BackendFoo defined after use

What is the reasoning behind this? It could be extremely useful to have 
this functionality. The only reason I can think of for doing this is 
that this code could be evaluated after modules that depend on the 
version being defined, causing it not to work. Surely there would be a 
way around that?

If it's not possible to fix this, what way would you recommend I get 
around this? The only idea I've come up with so far is to do:

else
{
   static assert( false, "Please compile with 
-version=Backend(Foo|Bar|Bar)" );
}

Which is less than optimal.