Comment 1 for bug 1368847

 I explored this case when fixing some bugs in the Google EVAL.
 The question is what it means "not to see" a thing that you are not supposed to look at, and unsurprisingly, Lisp implementations
 differ on this.
But (LOCALLY (DECLARE SPECIAL)) is the strangest of the strange cases.
Start with one that's not quite as strange-

(defun f ()
  (symbol-macrolet ((x ''foo))
    (let ((x 3))
      (macrolet ((z () x))
        (z)))))

In this example, the macro Z is using X in its own code and it sees, but should not see, a lexical variable of that name shadowing a lexical macro. SBCL's compiler does something different from other Lisps: we "hop over" the binding that you should not see, and we see the binding that you are permitted to see.
Run that through the compiler, and (F) => FOO.

But try it in brand X lisp and then brand Y lisp:

*** - Invalid access to the value of the lexical variable X from within a MACROLET definition
The following restarts are available:
ABORT :R1 Abort main loop

> Error: While compiling Z :
> Illegal reference to lexically defined variable X., in process listener(1).
> Type :GO to continue, :POP to abort, :R for a list of available restarts.
> If continued: continue compilation ignoring this form

In each case they've said here is something that we *know* you should not see, and "no" we're not going to look past that to see the thing that would be visible. Now suppose X were a bound special - I think the argument is the same - it should be an error.

So back to the strangest of the strange - a free special makes the macro know that there is something which is neither prohibited to see nor expressly permitted to see.
In this case, brand X lisp still tells you that it is illegal to see, and brand Y lisp tells you ' unbound variable '