Fix documentation of forward-line

* src/cmds.c (Fforward_line): Clarify the return value if the line
at end of accessible portion of the buffer has no newline.
* doc/lispref/positions.texi (Text Lines): Document what happens
if the line at end of accessible portion of buffer has no newline.

(Bug#20587)

2 files changed, 20 insertions, 8 deletions

diff --git a/doc/lispref/positions.texi b/doc/lispref/positions.texi
index e7c79d58241..c972bbb2e43 100644
--- a/doc/lispref/positions.texi
+++ b/doc/lispref/positions.texi
@@ -350,10 +350,11 @@ would move to.
 @deffn Command forward-line &optional count
 @cindex beginning of line
 This function moves point forward @var{count} lines, to the beginning of
-the line.  If @var{count} is negative, it moves point
-@minus{}@var{count} lines backward, to the beginning of a line.  If
-@var{count} is zero, it moves point to the beginning of the current
-line.  If @var{count} is @code{nil}, that means 1.
+the line following that.  If @var{count} is negative, it moves point
+@minus{}@var{count} lines backward, to the beginning of a line
+preceding that.  If @var{count} is zero, it moves point to the
+beginning of the current line.  If @var{count} is @code{nil}, that
+means 1.
 
 If @code{forward-line} encounters the beginning or end of the buffer (or
 of the accessible portion) before finding that many lines, it sets point
@@ -362,7 +363,11 @@ there.  No error is signaled.
 @code{forward-line} returns the difference between @var{count} and the
 number of lines actually moved.  If you attempt to move down five lines
 from the beginning of a buffer that has only three lines, point stops at
-the end of the last line, and the value will be 2.
+the end of the last line, and the value will be 2.  As an explicit
+exception, if the last accessible line is non-empty, but has no
+newline (e.g., if the buffer ends without a newline), the function
+sets point to the end of that line, and the value returned by the
+function counts that line as one line successfully moved.
 
 In an interactive call, @var{count} is the numeric prefix argument.
 @end deffn
diff --git a/src/cmds.c b/src/cmds.c
index b590805ea8a..6f9982eebb2 100644
--- a/src/cmds.c
+++ b/src/cmds.c
@@ -110,10 +110,17 @@ DEFUN ("forward-line", Fforward_line, Sforward_line, 0, 1, "^p",
 Precisely, if point is on line I, move to the start of line I + N
 \("start of line" in the logical order).
 If there isn't room, go as far as possible (no error).
+
 Returns the count of lines left to move.  If moving forward,
-that is N - number of lines moved; if backward, N + number moved.
-With positive N, a non-empty line at the end counts as one line
-successfully moved (for the return value).  */)
+that is N minus number of lines moved; if backward, N plus number
+moved.
+
+Exception: With positive N, a non-empty line at the end of the
+buffer, or of its accessible portion, counts as one line
+successfully moved (for the return value).  This means that the
+function will move point to the end of such a line and will count
+it as a line moved across, even though there is no next line to
+go to its beginning.  */)
   (Lisp_Object n)
 {
   ptrdiff_t opoint = PT, pos, pos_byte, shortage, count;