patch 8.2.1686: Vim9: "const!" not sufficiently tested

Problem:    Vim9: "const!" not sufficiently tested.
Solution:   Add a few more test cases.  Fix type checking.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 103e696..e8a3a21 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -5066,12 +5066,13 @@
 		    {
 			type_T *use_type = lvar->lv_type;
 
-			// without operator type is here, otherwise below
+			// without operator check type here, otherwise below
 			if (has_index)
 			{
 			    use_type = use_type->tt_member;
 			    if (use_type == NULL)
-				use_type = &t_void;
+				// could be indexing "any"
+				use_type = &t_any;
 			}
 			if (need_type(stacktype, use_type, -1, cctx, FALSE)
 								       == FAIL)