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Emacs authors decided to completely remove when-let and if-let in 26.1, and replaced them with when-let* and if-let* from subr-x.el

• Prior to emacs 26.1
  • there was no when-let* , if-let*.
  • but there was when-let and if-let
• In 26.1 and later
  • there is when-let*, if-let*
  • but there is no when-let, if-let

IMHO, this was sort of a silly, poorly thought-out obsolescence on the emacs hackers part, but thought you'd like to know.

They must have had their reasons. Maybe this thread will tell us why: https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00052.html

I didn't have an opportunity to read the entire thread.

You probably use an emacs version >= 26.1 for development.

Maybe an alias would work for the when-let*?

I quickly threw this together, but don't know how correct it is. It's not tested...

(require 'subr-x)

(when (or (< emacs-major-version 26)
          (and (= emacs-major-version 26) (< emacs-minor-version 1)))
  ;; `when-let' was summarily replaced with `when-let*' in emacs 26.1
  ;; with no define-obsolete-function-alias  so other developers are forced to use when-let*
  ;; leaving users of older emacs versions in a lurch.
  (defalias 'when-let* 'when-let ))

The other option is just to increase the ;; Package-Requires to ((emacs "26.1"))