does cast make an lvalue appear to be an rvalue

[Daniel Davidson]

The code below fails to compile due to the last line. I was 
hoping casting away immutable would allow the call to foo. I 
think it is not accepted because of the rval to ref issue. If 
that is the case, how can foo be called by casting?

I'm not a fan of casting but I'm finding cases where it is the 
only recourse to create immutable data using impure functions 
that should be pure.

Thanks
Dan

import std.conv;

struct T {
   int[] i;
   string[string] ss;
}

void foo(ref T t) {
}

void main() {
   T t1;
   auto t2 = immutable T();
   foo(t1);
   foo(cast()t2);
}