cgen: fix struct field array index error (#14417)

vlib/v/gen/c/index.v

@@ -172,8 +172,7 @@ fn (mut g Gen) index_of_array(node ast.IndexExpr, sym ast.TypeSymbol) {
 	}
 	// `vals[i].field = x` is an exception and requires `array_get`:
 	// `(*(Val*)array_get(vals, i)).field = x;`
-	is_selector := node.left is ast.SelectorExpr
+	if g.is_assign_lhs && node.is_setter {
-	if g.is_assign_lhs && !is_selector && node.is_setter {
 		is_direct_array_access := (g.fn_decl != 0 && g.fn_decl.is_direct_arr) || node.is_direct
 		is_op_assign := g.assign_op != .assign && info.elem_type != ast.string_type
 		if is_direct_array_access {

vlib/v/tests/struct_field_array_index_test.v

@@ -0,0 +1,17 @@
+struct App {
+mut:
+	buffer []string
+}
+
+fn test_struct_field_array_index() {
+	mut app := &App{
+		buffer: []string{len: 2}
+	}
+
+	app.buffer[0] += 'hello'
+	app.buffer[1] += 'world'
+
+	println(app)
+
+	assert app.buffer == ['hello', 'world']
+}