abbrev--default-expand is a byte-compiled function defined in
abbrev.el.gz.

Signature
(abbrev--default-expand)

Documentation
Default function to use for abbrev-expand-function.

This also respects the obsolete wrapper hook abbrev-expand-functions.
(See with-wrapper-hook for details about wrapper hooks.)
Calls abbrev-insert to insert any expansion, and returns what it does.

This function has :around advice: oo--ensure-self-insert.
This function has :around advice: oo--add-period-maybe.
This function has :around advice: oo--pulse-expansion.

References
References in abbrev.el.gz:
(defvar abbrev-expand-function ...) 1 reference

Find all references Functions used by abbrev--default-expand

Advice
This function is advised.

Debugging
Enable edebug Enable tracing
Disassemble Forget

Source Code
;; Defined in /usr/share/emacs/29.4/lisp/abbrev.el.gz
(defun abbrev--default-expand ()
  "Default function to use for `abbrev-expand-function'.
This also respects the obsolete wrapper hook `abbrev-expand-functions'.
\(See `with-wrapper-hook' for details about wrapper hooks.)
Calls `abbrev-insert' to insert any expansion, and returns what it does."
  (subr--with-wrapper-hook-no-warnings abbrev-expand-functions ()
    (pcase-let ((`(,sym ,name ,wordstart ,wordend) (abbrev--before-point)))
      (when sym
        (let ((startpos (copy-marker (point) t))
              (endmark (copy-marker wordend t)))
          (unless (or ;; executing-kbd-macro
                   noninteractive
                   (window-minibuffer-p))
            ;; Add an undo boundary, in case we are doing this for
            ;; a self-inserting command which has avoided making one so far.
            (undo-boundary))
          ;; Now sym is the abbrev symbol.
          (setq last-abbrev-text name)
          (setq last-abbrev sym)
          (setq last-abbrev-location wordstart)
          ;; If this abbrev has an expansion, delete the abbrev
          ;; and insert the expansion.
          (prog1
              (abbrev-insert sym name wordstart wordend)
            ;; Yuck!!  If expand-abbrev is called with point slightly
            ;; further than the end of the abbrev, move point back to
            ;; where it started.
            (if (and (> startpos endmark)
                     (= (point) endmark)) ;Obey skeletons that move point.
                (goto-char startpos))))))))

Symbol Properties
defalias-fset-function
  #[128 "\304\300\301#\207"
        [advice--defalias-fset nil :around nil apply]
        5 advice]
  Disassemble
function-documentation
  (advice--make-docstring 'abbrev--default-expand)