bug#59674: 28.2; icomplete-force-complete Incorrectly Completes Filename Case

[Cameron Norton]

Using icomplete, I often navigate to certain files by using a single
character for each component of the path name before completing with
icomplete-force-complete, which I have bound to <tab> while in the
minibuffer. So a file

./foo/bar/baz/asdf.txt

would be visited via the sequence

C-x C-f f/b/b/a <tab> <return>

Using icomplete-force-complete this way emulates the <tab> behavior in
completion packages like Vertico without the extra dependency.

However, with completion case sensitivity disabled, this sometimes
results in the file's path being completed incorrectly with regards to
capitalization. This is very noticeable when finding files in the usual
subdirectories of $HOME, with ~/doc <tab> completing to ~/documents/
rather than ~/Documents/, for example.

I can consistently reproduce this behavior through these steps. I have
done this on both my usual Emacs 28.2 as well as the git master
(currently 7939184).

 1. Start a clean emacs via emacs -Q.

 2. Evaluate the following:
      (icomplete-mode)
      (setq read-file-name-completion-ignore-case t)

 3. Navigate to some subdirectory of $HOME via M-x cd or equivalent.

 4. Start using C-x C-f to find a file in a different subdirectory of
    $HOME. The target file should be located in a subdirectory that
    starts with a capital letter (e.g. start finding something under
    ~/Documents somewhere).

 5. Enter the file's pathname relative to the home directory (starting
    with ~/) or an abbreviation like above using all lowercase
    letters. Do not use any completion while doing so, and do not clear
    the minibuffer of the current non-home directory beforehand.

 6. Complete the whole filename using icomplete-force-complete, which is
    bound to C-M-i by default.

An example run on my machine has me cd to ~/Downloads

M-x cd <return> ~/Downloads <return>

then try to find the file ~/Documents/Misc/asdf.org by entering

C-x C-f ~/d/m/a C-M-i

This instead completes to ~/documents/Misc/asdf.org, with "Documents"
uncapitalized but the other components completing with proper
capitalization.

(Apologies if this sent twice, I'm having unrelated technical issues.)

Cameron Norton

---

In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.34, cairo 
version 1.17.6)
 of 2022-09-12 built on frederik
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Arch Linux

Configured using:
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Load-path shadows:
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