UFCS on forward reference

[John Belmonte]

C API's often use a opaque struct pointer as a handle.  Mapping 
such a struct to D using a forward declaration, I noticed that 
UFCS doesn't work:

   struct State;
   ...
   State* s = new_state();
   foo(s);  // ok
   s.foo(); // compile error

Error detail:

   Error: struct State is forward referenced when looking for 'foo'
   Error: struct State is forward referenced when looking for 
'opDot'
   Error: struct State is forward referenced when looking for 
'opDispatch'

I'm wondering if anything would be harmed by removing this 
restriction.

As a workaround I can use "struct State {}", but that feels wrong.