Optional type - how to correctly reset a wrapped immutable T

[SimonN]

On Monday, 26 March 2018 at 14:17:03 UTC, Jonathan M Davis wrote:
> Rebindable does is pretty questionable as far as the type 
> system goes, but it does what it does by forcing pointer 
> semantics on a class reference, so the point is arguable.

Yeah, I've always assumed that Rebindable cannot be implemented 
without internally breaking the type system, then exposing a safe 
interface.

But this sparked my interest, I've dug out the Rebindable code:

     private mixin template RebindableCommon(T, U, alias This)
         if (is(T == class) || is(T == interface) || 
isAssociativeArray!T)
     {
         private union
         {
             T original; // e.g., immutable(A) for classs A
             U stripped; // the unqualified type, e.g., A
         }
         // ...
     }

Does Rebindable-using code, oblivious of the hacks inside 
Rebindable, remain 100 % safe even with aggressive compiler 
optimizations? For class A, inside Rebindable!(immutable A), 
there is a union of (immutable A) and A. I suspect that the D 
compiler is allowed to treat this reference to (immutable A) as 
immutable itself. Have there never been bugs here when, later, 
stripped = another.stripped;?

-- Simon