Behaviour of (letrec ([x x]) x) changed in Racket

[soegaard]

The latest Racket has changed the behaviour of (letrec ([x x]) x).
Currently any program beginning with #lang whalesong fails due
to an "x undefined" error. The undefined value is now exported
from racket/undefined.

[soegaard]

Note that (letrec ([x x]) x) is used in other files too:

grep -r undefined * | grep letrec
whalesong/cs019/private/sigs-patched.rkt:(define the-undefined-value (letrec ([x x]) x))
whalesong/cs019/teach-runtime.rkt:(define undefined (letrec ([x x]) x))
whalesong/tests/older-tests/moby-programs/misc.rkt:(check-expect (undefined? (letrec ([x x]) x)) true)

[soegaard]

Also in kernel.rkt the function -undefined? could be replaced with:
(require racket/undefined)
(define (-undefined? x)
(eq? x undefined))