bug#63535: Master branch: Error in forw_comment (syntax.c) handling of escaped LFs

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

In the master branch:

Consider the following C++ Mode buffer:

    // comment \
    comment line 2
    line_3();

..  The backslash at the end of line 1 extends the comment into line 2.

Put point at // on L1, and do:

    M-: (setq s (parse-partial-sexp (point) (+ (point) 9)))

..  s gets the parse state of the inside of the comment.

Now put point at EOL 1, between the backslash and the LF.  Do

    M-: (parse-partial-sexp (point) (point-max) nil nil s 'syntax-table)

..  This ought to leave point at BOL 2, since the syntax before the LF at
EOL 1 is that of a C++ comment, otherwise neutral.  Instead, it leaves
point wrongly at BOL 3.

#########################################################################

The reason for this bug is at L+42 of forw_comment (in syntax.c).  There
we have

   && !(comment_end_can_be_escaped && char_quoted (from, from_byte))

..  Checking char_quoted is wrong.  Instead the function should check the
current parse state.

-- 
Alan Mackenzie (Nuremberg, Germany).