How to bind C-[ for real (seriously for real this time)

Question

Is there any way to bind C-[ to something and not have all M- (Meta) bindings messed up?

Not the first time this question comes up. Unfortunately, the only solution offered in the previous thread is a) Linux specific, b) requires an external tool (again Linux specific) that has nothing to do with Emacs. In the same vein I could have used something like Karabiner on a OS X to steal the C-[ sequence before Emacs ever gets it but this is clunky and expensive.

Given that it is Emacs that insists on translating C-[ to ESC, C-i to TAB and probably others I would very much like to break these relationships and get precious key sequences back. Recently I ran full speed into this problem and blamed quiet innocent smartparens mode.

A possible solution that involved function-key-map and key-translation-map was discussed here but alas it either no longer works or did not work in the first place (or I'm doing something wrong). I think it was going in the right direction though.

Answer 1

Adapted from my own config:

(define-key input-decode-map [?\C-\[] (kbd "<C-[>"))
(global-set-key (kbd "<C-[>") 'butterfly)

This will obviously only work in the GUI.

edit: Note that input-decode-map is terminal-local which means modifying it won't work if you're using emacsclient, but will do if you're using emacs. I've fixed the issue in my config by wrapping it in a command operating on a frame and adding it to the after-make-frame-functions hook.