I want to try new features in emacs 26, I'm using emacs 27, generators and threads, but when I try to use on ielm buffer, I get the same error:
ELISP> (iter-defun my-iter (x)
(iter-yield (1+ (iter-yield (1+ x))))
;; Return normally
-1)
*** Eval error *** Assertion failed: lexical-binding
This example is from ergoemacs