vim-patch:8.1.1988: :startinsert! does not work the same way as "A"

Problem:    :startinsert! does not work the same way as "A".
Solution:   Use the same code to move the cursor. (closes vim/vim#4896)
8d3b51084a

src/nvim/ex_docmd.c

@@ -8305,15 +8305,14 @@ static void ex_startinsert(exarg_T *eap)
     if (!curwin->w_cursor.lnum) {
       curwin->w_cursor.lnum = 1;
     }
-    coladvance((colnr_T)MAXCOL);
-    curwin->w_curswant = MAXCOL;
-    curwin->w_set_curswant = FALSE;
+    set_cursor_for_append_to_line();
   }
 
-  /* Ignore the command when already in Insert mode.  Inserting an
-   * expression register that invokes a function can do this. */
-  if (State & INSERT)
+  // Ignore the command when already in Insert mode.  Inserting an
+  // expression register that invokes a function can do this.
+  if (State & INSERT) {
     return;
+  }
 
   if (eap->cmdidx == CMD_startinsert)
     restart_edit = 'a';
@@ -8325,7 +8324,7 @@ static void ex_startinsert(exarg_T *eap)
   if (!eap->forceit) {
     if (eap->cmdidx == CMD_startinsert)
       restart_edit = 'i';
-    curwin->w_curswant = 0;         /* avoid MAXCOL */
+    curwin->w_curswant = 0;  // avoid MAXCOL
   }
 
   if (VIsual_active) {

src/nvim/normal.c

@@ -7508,6 +7508,23 @@ static void nv_esc(cmdarg_T *cap)
     restart_edit = 'a';
 }
 
+// Move the cursor for the "A" command.
+void set_cursor_for_append_to_line(void)
+{
+  curwin->w_set_curswant = true;
+  if (ve_flags == VE_ALL) {
+    const int save_State = State;
+
+    // Pretend Insert mode here to allow the cursor on the
+    // character past the end of the line
+    State = INSERT;
+    coladvance((colnr_T)MAXCOL);
+    State = save_State;
+  } else {
+    curwin->w_cursor.col += (colnr_T)STRLEN(get_cursor_pos_ptr());
+  }
+}
+
 /// Handle "A", "a", "I", "i" and <Insert> commands.
 static void nv_edit(cmdarg_T *cap)
 {
@@ -7529,18 +7546,8 @@ static void nv_edit(cmdarg_T *cap)
     clearop(cap->oap);
   } else if (!checkclearopq(cap->oap)) {
     switch (cap->cmdchar) {
-    case 'A':           /* "A"ppend after the line */
-      curwin->w_set_curswant = true;
-      if (ve_flags == VE_ALL) {
-        int save_State = State;
-
-        /* Pretend Insert mode here to allow the cursor on the
-         * character past the end of the line */
-        State = INSERT;
-        coladvance((colnr_T)MAXCOL);
-        State = save_State;
-      } else
-        curwin->w_cursor.col += (colnr_T)STRLEN(get_cursor_pos_ptr());
+    case 'A':           // "A"ppend after the line
+      set_cursor_for_append_to_line();
       break;
 
     case 'I':           /* "I"nsert before the first non-blank */

src/nvim/testdir/test_edit.vim

@@ -1484,3 +1484,18 @@ func Test_edit_special_chars()
 
   close!
 endfunc
+
+func Test_edit_startinsert()
+  new
+  set backspace+=start
+  call setline(1, 'foobar')
+  call feedkeys("A\<C-U>\<Esc>", 'xt')
+  call assert_equal('', getline(1))
+
+  call setline(1, 'foobar')
+  call feedkeys(":startinsert!\<CR>\<C-U>\<Esc>", 'xt')
+  call assert_equal('', getline(1))
+
+  set backspace&
+  bwipe!
+endfunc