Distinguish buttons from widgets (bug#34506)

* lisp/button.el (button-at):
* lisp/wid-edit.el (widget-at): Avoid returning a false positive
when looking for a button and finding a widget, or vice versa.
* test/lisp/button-tests.el:
* test/lisp/wid-edit-tests.el: New files.

lisp/button.el

@@ -382,10 +382,12 @@ Also see `make-text-button'."
 If the button at POS is a text property button, the return value
 is a marker pointing to POS."
   (let ((button (get-char-property pos 'button)))
-    (if (or (overlayp button) (null button))
-	button
-      ;; Must be a text-property button; return a marker pointing to it.
-      (copy-marker pos t))))
+    (and button (get-char-property pos 'category)
+         (if (overlayp button)
+             button
+           ;; Must be a text-property button;
+           ;; return a marker pointing to it.
+           (copy-marker pos t)))))
 
 (defun next-button (pos &optional count-current)
   "Return the next button after position POS in the current buffer.

lisp/wid-edit.el

@@ -1163,8 +1163,9 @@ When not inside a field, signal an error."
 
 (defun widget-at (&optional pos)
   "The button or field at POS (default, point)."
-  (or (get-char-property (or pos (point)) 'button)
-      (widget-field-at pos)))
+  (let ((widget (or (get-char-property (or pos (point)) 'button)
+                    (widget-field-at pos))))
+    (and (widgetp widget) widget)))
 
 ;;;###autoload
 (defun widget-setup ()

test/lisp/button-tests.el

+;;; button-tests.el --- tests for button.el -*- lexical-binding: t -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest button-at ()
+  "Test `button-at' behavior."
+  (with-temp-buffer
+    (should-not (button-at (point)))
+    (let ((button (insert-text-button "text button"))
+          (marker (button-at (1- (point)))))
+      (should (markerp marker))
+      (should (= (button-end button) (button-end marker) (point))))
+    (let ((button  (insert-button "overlay button"))
+          (overlay (button-at (1- (point)))))
+      (should (overlayp overlay))
+      (should (eq button overlay)))
+    ;; Buttons and widgets are incompatible (bug#34506).
+    (widget-create 'link "link widget")
+    (should-not (button-at (1- (point))))))
+
+;;; button-tests.el ends here

test/lisp/wid-edit-tests.el

+;;; wid-edit-tests.el --- tests for wid-edit.el -*- lexical-binding: t -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'wid-edit)
+
+(ert-deftest widget-at ()
+  "Test `widget-at' behavior."
+  (with-temp-buffer
+    (should-not (widget-at))
+    (let ((marco (widget-create 'link "link widget"))
+          (polo  (widget-at (1- (point)))))
+      (should (widgetp polo))
+      (should (eq marco polo)))
+    ;; Buttons and widgets are incompatible (bug#34506).
+    (insert-text-button "text button")
+    (should-not (widget-at (1- (point))))
+    (insert-button "overlay button")
+    (should-not (widget-at (1- (point))))))
+
+;;; wid-edit-tests.el ends here