checker: fix compiler panic for for k, mut val in app.field[x].values { (#17468)

vlib/v/checker/for.v

@@ -195,15 +195,18 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 						c.error('map literal is immutable, it cannot be changed', node.cond.pos)
 					}
 					ast.SelectorExpr {
-						root_ident := node.cond.root_ident() or { node.cond.expr as ast.Ident }
+						if root_ident := node.cond.root_ident() {
 							if root_ident.kind != .unresolved {
-							if !(root_ident.obj as ast.Var).is_mut {
+								if var := node.scope.find_var(root_ident.name) {
+									if !var.is_mut {
 										sym2 := c.table.sym(root_ident.obj.typ)
 										c.error('field `${sym2.name}.${node.cond.field_name}` is immutable, it cannot be changed',
 											node.cond.pos)
 									}
 								}
 							}
+						}
+					}
 					else {}
 				}
 			} else if node.val_is_ref {

vlib/v/tests/for_in_mut_mutable_app_field_test.v

@@ -0,0 +1,20 @@
+struct Item {
+mut:
+	field []int
+}
+
+struct App {
+mut:
+	index [][]Item
+}
+
+fn test_for_in_struct_with_mutable_array_field_indexed_several_times_in_the_loop_condition_part() {
+	mut app, x, y := App{[[Item{
+		field: [1, 2, 3]
+	}]]}, 0, 0
+	for i, mut v in app.index[x][y].field {
+		assert v != 0
+		v = 555
+	}
+	assert app.index[x][y] == Item{[555, 555, 555]}
+}