Improve accuracy of line/column numbers in byte compiler's warning messages.

* lisp/emacs-lisp/bytecomp.el (byte-compile-set-symbol-position): ensure new
value of byte-compile-last-position is not lower than old value.
(byte-compile-function-warn): call byte-compile-set-symbol-position.

lisp/emacs-lisp/bytecomp.el

@@ -1022,39 +1022,42 @@ Each function's symbol gets added to `byte-compile-noruntime-functions'."
 	(setcdr list (cddr list)))
       total)))
 
-;; The purpose of this function is to iterate through the
+;; The purpose of `byte-compile-set-symbol-position' is to attempt to
-;; `read-symbol-positions-list'.  Each time we process, say, a
+;; set `byte-compile-last-position' to the "current position" in the
-;; function definition (`defun') we remove `defun' from
+;; raw source code.  This is used for warning and error messages.
-;; `read-symbol-positions-list', and set `byte-compile-last-position'
-;; to that symbol's character position.  Similarly, if we encounter a
-;; variable reference, like in (1+ foo), we remove `foo' from the
-;; list.  If our current position is after the symbol's position, we
-;; assume we've already passed that point, and look for the next
-;; occurrence of the symbol.
 ;;
-;; This function should not be called twice for the same occurrence of
+;; The function should be called for most occurrences of symbols in
-;; a symbol, and it should not be called for symbols generated by the
+;; the forms being compiled, strictly in the order they occur in the
-;; byte compiler itself; because rather than just fail looking up the
+;; source code.  It should never be called twice for any single
-;; symbol, we may find an occurrence of the symbol further ahead, and
+;; occurrence, and should not be called for symbols generated by the
-;; then `byte-compile-last-position' as advanced too far.
+;; byte compiler itself.
 ;;
-;; So your're probably asking yourself: Isn't this function a
+;; The function works by scanning the elements in the alist
-;; gross hack?  And the answer, of course, would be yes.
+;; `read-symbol-positions-list' for the next match for the symbol
+;; after the current value of `byte-compile-last-position', setting
+;; that variable to the match's character position, then deleting the
+;; matching element from the list.  Thus the new value for
+;; `byte-compile-last-position' is later than the old value unless,
+;; perhaps, ALLOW-PREVIOUS is non-nil.
+;;
+;; So your're probably asking yourself: Isn't this function a gross
+;; hack?  And the answer, of course, would be yes.
 (defun byte-compile-set-symbol-position (sym &optional allow-previous)
   (when byte-compile-read-position
-    (let (last entry)
+    (let ((last byte-compile-last-position)
+          entry)
       (while (progn
-	       (setq last byte-compile-last-position
+	       (setq entry (assq sym read-symbol-positions-list))
-		     entry (assq sym read-symbol-positions-list))
 	       (when entry
 		 (setq byte-compile-last-position
 		       (+ byte-compile-read-position (cdr entry))
 		       read-symbol-positions-list
 		       (byte-compile-delete-first
 			entry read-symbol-positions-list)))
+	       (and entry
                     (or (and allow-previous
                              (not (= last byte-compile-last-position)))
-		   (> last byte-compile-last-position)))))))
+                        (> last byte-compile-last-position))))))))
 
 (defvar byte-compile-last-warned-form nil)
 (defvar byte-compile-last-logged-file nil)
@@ -1284,6 +1287,7 @@ when printing the error message."
 	(t (format "%d-%d" (car signature) (cdr signature)))))
 
 (defun byte-compile-function-warn (f nargs def)
+  (byte-compile-set-symbol-position f)
   (when (get f 'byte-obsolete-info)
     (byte-compile-warn-obsolete f))