bug#49739: 28.0.50; byte-compiled pcase pattern match failure

[No Wayman <iarchivedmywholelife@gmail.com>]

Recently a user reported a bug on Org-mode's mailing list in which 
an org-capture template was failing to fill properly:

https://orgmode.org/list/87fsw3zyjl.fsf@ucl.ac.uk/T/#t

I was able to reproduce the issue with emacs -Q and the following 
recipe:

Evaluate the following in *scratch*:

(progn
  (require 'org-capture)
  (org-capture-fill-template "%^t"))

This should prompt the user for a time, instead we hit the `_` 
case in one of the pcase calls in `org-capture-fill-template' 
despite the valid template string. This results in the error:

"unrecognized template placeholder: %^t"

I've observed that this only occurs when 
`org-capture-fill-template' has been byte-compiled.
Re-evaluating the defun results in the expected behavior. And 
byte-compiling the function puts us back in the erroneous state:

(progn
  (byte-compile #'org-capture-fill-template)
  (org-capture-fill-template "%^t")) ; unrecognized template 
  placeholder...

I didn't do a proper git-bisect, but the everything works as 
expected when building from the parent 
(3af2cee64b86e4ce59adb8e8720d92db35039cbc) of the following 
commit:

commit 937b6c18bd6c4806eb1e4c8764db56b314c09056
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Date:   Fri Mar 19 17:42:22 2021 -0400

    * lisp/emacs-lisp/pcase.el (pcase-compile-patterns): New 
    function (bug#47261)
    
    Extracted from `pcase--expand`.
    (pcase--expand): Use it.

I tried to do some more debugging, but I'm not familiar with how 
to debug a problem which only crops up in byte-compiled elisp. 
Happy to provide any other information if necessary.

~ Nick