bug#23443: 25.0.93; Temporary change to syntax-table is in effect during syntax-propertize

[Noam Postavsky <npostavs@users.sourceforge.net>]

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On the emacs-25 and master branches, changing the syntax-table
temporarily affects the table used during syntax-propertize. This does
not happen in 24.5 (unless the difference is just a timing thing?).
This triggers an infinite loop when expanding certain snippets with
yasnippet in haskell-mode (see [1] and [2]).

Attached is a simplified reproducer, run as

    emacs -Q -l synprop-bug.el

In Emacs 25+, this results in the message

Error during redisplay: (internal--syntax-propertize 2) signaled
(error "syntax table is changed to standard-syntax-table during syntax
propertize")


In Emacs 24.5, it runs without a problem. It's not entirely clear to
me who should be reponsible for ensuring the right syntax-table during
syntax-propertize.

[1]: https://github.com/capitaomorte/yasnippet/issues/687
[2]: https://github.com/haskell/haskell-mode/issues/1315

[-- Attachment #2: synprop-bug.el --]
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(defun test-check-syntax-table (begin end)
  (unless (eq (syntax-table) test-syntax-prop-mode-syntax-table)
    (error "syntax table is changed to %s during syntax propertize"
           (if (eq (syntax-table) (standard-syntax-table))
               'standard-syntax-table
             'other-syntax-table))))

(define-derived-mode test-syntax-prop-mode prog-mode "TestSyntaxProp"
  "A mode for testing syntax-propertize bug(?)."
  (setq-local syntax-propertize-function #'test-check-syntax-table)
  (setq-local font-lock-defaults
              '(nil nil nil nil nil
                (font-lock-syntactic-face-function . ignore)
                (parse-sexp-lookup-properties . t))))

(switch-to-buffer "tt")
(test-syntax-prop-mode)
(erase-buffer)
(insert "foo")
(with-syntax-table (standard-syntax-table)
  (scan-sexps 1 1))