Trouble binding C-return: C- must prefix a single character

Question

This is what I have:

(add-hook 'magit-status-mode-hook
  (lambda ()
    (local-set-key (kbd "<C-return>") 'magit-diff-visit-file-other-window)))

Opening a magit-status buffer fails with "C- must prefix a single character, not return".

There are several related questions, but their solutions (for example moving the <> around return instead of the whole string) don't work. Using RET instead of return doesn't work either.

Emacs 26.1

Answer 1

In the comments we established that the problem wasn't the code shown in the question:

(add-hook 'magit-status-mode-hook
  (lambda ()
    (local-set-key (kbd "<C-return>") 'magit-diff-visit-file-other-window)))

The problem was the use of this pattern:

(add-hook 'HOOK (lambda ...))

When that pattern is used, updating the code of that (lambda...) function necessitates explicitly removing the original/bad lambda from the hook variable before adding a new updated version -- otherwise both functions will be called when the hook runs.

So the problem was that an earlier attempt (with a bad key binding syntax) was still being executed.

Inspecting C-hv magit-status-mode-hook showed that the old lambda was still present.

---

The better pattern is:

(add-hook 'magit-status-mode-hook 'my-magit-status-mode-hook)

(defun my-magit-status-mode-hook ()
  "Custom `magit-status-mode' behaviours."
  (local-set-key (kbd "<C-return>") 'magit-diff-visit-file-other-window))

As the named function can subsequently be updated without also needing to touch the hook variable.

Answer 2

I had this issue as well when I was trying to use C-<return> for another command in elpy. I managed to resolve it by first looking up the global key binding for C-<return> (try C-h k then type C-<return> to see what Emacs thinks is assigned to that key). In my situation, C-<return> was bound to CUA mode.

Then go into your configuration file and first set that binding to nil. In my situation, it looked like this:

;; Add keybindings for python that override CUA mode rect
(define-key cua-global-keymap [C-return] nil)
(global-set-key (kbd "C-RET") 'elpy-shell-send-statement-and-step)