bug#22983: syntax-ppss returns wrong result.

[Alan Mackenzie]

Hello, Emacs.

The fundamental contract in syntax-ppss is that (syntax-ppss POS)
returns the same value as (parse-partial-sexp (point-min) POS) (with the
exception of elements 2 and 6).  This is currently not always the case.

In the master branch, emacs -Q and visit xdisp.c with C-x C-f.  Follow
this recipe:

    M-: (syntax-ppss-flush-cache 1)
    M-: (setq ppss-0 (syntax-ppss 40000))
    M-<
    C-s #include " <CR>
    M->
    C-x n n
    M-: (setq ppss-1 (syntax-ppss 40000))
    M-: (setq parse (parse-partial-sexp (point-min) 40000))

At this point, `ppss-1' and `parse' should match (apart from elements 2
and 6).  What we actually have is:

    ppss-1: (2 39992 nil nil nil nil 2 nil nil (39975 39992))
    parse:  (0 nil 15674 34 nil nil 0 nil 15675 nil)

.

-- 
Alan Mackenzie (Nuremberg, Germany).