patch 8.2.3336: behavior of negative index in list change changed

Problem:    Behavior of negative index in list change changed. (Naruhiko
            Nishino)
Solution:   Only change it for Vim9 script. (closes #8749)

src/list.c

@@ -1146,15 +1146,19 @@ list_slice_or_index(
 	n1 = len + n1;
     if (n1 < 0 || n1 >= len)
     {
-	// For a range we allow invalid values and return an empty
-	// list.  A list index out of range is an error.
+	// For a range we allow invalid values and for legacy script return an
+	// empty list, for Vim9 script start at the first item.
+	// A list index out of range is an error.
 	if (!range)
 	{
 	    if (verbose)
 		semsg(_(e_listidx), (long)n1_arg);
 	    return FAIL;
 	}
+	if (in_vim9script())
 	    n1 = n1 < 0 ? 0 : len;
+	else
+	    n1 = len;
     }
     if (range)
     {

src/testdir/test_listdict.vim

@@ -42,6 +42,23 @@ func Test_list_slice()
   let l[:1] += [1, 2]
   let l[2:] -= [1]
   call assert_equal([2, 4, 2], l)
+
+  let lines =<< trim END
+      VAR l = [1, 2]
+      call assert_equal([1, 2], l[:])
+      call assert_equal([2], l[-1 : -1])
+      call assert_equal([1, 2], l[-2 : -1])
+  END
+  call CheckLegacyAndVim9Success(lines)
+
+  let l = [1, 2]
+  call assert_equal([], l[-3 : -1])
+
+  let lines =<< trim END
+      var l = [1, 2]
+      assert_equal([1, 2], l[-3 : -1])
+  END
+  call CheckDefAndScriptSuccess(lines)
 endfunc
 
 " List identity

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3336,
 /**/
     3335,
 /**/