Preserve point position after adding warning overlay

Resolves part of:
idris-community/idris2-mode#36
> "The cursor still jumps to the beginning of the line"

idris-tests.el

@@ -289,6 +289,36 @@ myReverse xs = revAcc [] xs where
       (kill-buffer))
     (idris-quit)))
 
+(ert-deftest idris-test-warning-overlay ()
+  "Test that `idris-warning-overaly-point' works as expected."
+  (let* ((buffer (find-file-noselect "test-data/AddClause.idr"))
+         (warning '("AddClause.idr" (5 7) (5 17) "Some warning message" ()))
+         (idris-raw-warnings '())
+         (idris-process-current-working-directory (file-name-directory (buffer-file-name buffer)))
+         (expected-position)
+         (expected-overlay))
+    (with-current-buffer buffer
+      (goto-char (point-min))
+      (re-search-forward "data Test")
+      (setq expected-position (point))
+
+      (idris-warning-overlay warning)
+
+      ;; Assert that the point position does not change
+      ;; https://github.com/idris-community/idris2-mode/issues/36
+      (should (eq (point) expected-position))
+
+      ;; Assert side effect
+      (should (not (null idris-raw-warnings)))
+
+      ;; Assert that overlay was added
+      (setq expected-overlay (car (overlays-in (point-min) (point-max))))
+      (should (not (null expected-overlay)))
+      (should (string= (overlay-get expected-overlay 'help-echo)
+                       "Some warning message"))
+      ;; Cleanup
+      (kill-buffer))))
+
 (ert-deftest idris-backard-toplevel-navigation-test-2pTac9 ()
   "Test idris-backard-toplevel navigation command."
   (idris-test-with-temp-buffer

idris-warnings.el

@@ -94,24 +94,25 @@ is mostly the same as (startline startcolumn)"
              (buffer (get-file-buffer fullpath)))
         (when (not (null buffer))
           (with-current-buffer buffer
-            (save-restriction
-              (widen) ;; Show errors at the proper location in narrowed buffers
-              (goto-char (point-min))
-              (let* ((startp (line-beginning-position startline))
-                     (endp (line-end-position startline))
-                     (start (+ startp startcol))
-                     (end (if (and (= startline endline) (= startcol endcol))
-                              ;; this is a hack to have warnings reported which point to empty lines
-                              (if (= startp endp)
-                                  (progn (goto-char startp)
-                                         (insert " ")
-                                         (1+ endp))
-                                endp)
-                            (+ (line-beginning-position endline) endcol)))
-                     (overlay (idris-warning-overlay-at-point startp)))
-                (if overlay
-                    (idris-warning-merge-overlays overlay message)
-                  (idris-warning-create-overlay start end message))))))))))
+            (save-excursion
+              (save-restriction
+                (widen) ;; Show errors at the proper location in narrowed buffers
+                (goto-char (point-min))
+                (let* ((startp (line-beginning-position startline))
+                       (endp (line-end-position startline))
+                       (start (+ startp startcol))
+                       (end (if (and (= startline endline) (= startcol endcol))
+                                ;; a hack to have warnings, which point to empty lines, reported
+                                (if (= startp endp)
+                                    (progn (goto-char startp)
+                                           (insert " ")
+                                           (1+ endp))
+                                  endp)
+                              (+ (line-beginning-position endline) endcol)))
+                       (overlay (idris-warning-overlay-at-point startp)))
+                  (if overlay
+                      (idris-warning-merge-overlays overlay message)
+                    (idris-warning-create-overlay start end message)))))))))))
 
 (defun idris-warning-merge-overlays (overlay message)
   (overlay-put overlay 'help-echo