Fix syntax highlighting after string literal concat in python-mode

* lisp/progmodes/python.el (python-syntax-stringify): Fix
incorrect font-lock after string literal concatenation.
(Bug#45897)

* test/lisp/progmodes/python-tests.el
(python-font-lock-string-literal-concatenation): New test.

Co-authored-by: kobarity <kobarity@gmail.com>
Copyright-paperwork-exempt: yes

1 files changed, 3 insertions, 0 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 41f612c8b1c..9d840efb9da 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -909,6 +909,7 @@ is used to limit the scan."
   "Put `syntax-table' property correctly on single/triple quotes."
   (let* ((ppss (save-excursion (backward-char 3) (syntax-ppss)))
          (string-start (and (eq t (nth 3 ppss)) (nth 8 ppss)))
+         (string-literal-concat (numberp (nth 3 ppss)))
          (quote-starting-pos (- (point) 3))
          (quote-ending-pos (point)))
     (cond ((or (nth 4 ppss)             ;Inside a comment
@@ -921,6 +922,8 @@ is used to limit the scan."
           ((nth 5 ppss)
            ;; The first quote is escaped, so it's not part of a triple quote!
            (goto-char (1+ quote-starting-pos)))
+          ;; Handle string literal concatenation (bug#45897)
+          (string-literal-concat nil)
           ((null string-start)
            ;; This set of quotes delimit the start of a string.  Put
            ;; string fence syntax on last quote. (bug#49518)