No error signaled when binding constants with PROGV

[paulapatience]

clasp-2.7.0-249-g0434f374d (latest as of a few days ago) happily runs the following:

(progv (list nil) '()) ; => NIL
(progv (list nil) '(t)) ; => NIL
(progv (list t) '()) ; => NIL
(progv (list t) '(t)) ; => NIL

SBCL, CCL and ECL signal an error, and Clasp probably should too. (ABCL behaves like Clasp, incidentally.)

[paulapatience]

The issue is actually that binding constants in general is not prohibited:

(let ((nil 1)
      (t 2)     
      (pi 3))
  (list (list) nil t pi)) ; => (1 1 2 3)