checker: fix compiler panic for `for k, mut val in app.field[x].value…

…s {` (#17468)

vlib/v/checker/for.v

@@ -195,12 +195,15 @@ 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.kind != .unresolved {
-							if !(root_ident.obj as ast.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)
+						if root_ident := node.cond.root_ident() {
+							if root_ident.kind != .unresolved {
+								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)
+									}
+								}
 							}
 						}
 					}

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]}
+}