A more general idea involving ast composition

ast_macro.php

<?php

// User land implementation of generics using "AST composition"

// {% <code> %} would be a "quasi code" construct used to build AST objects
// that can be conveniently post-processed and lazily executed

function new_list(Php\Ast $type) : obj {
    // The anonymous class ast to be used as a template
    $ast = {%
        new class {
            protected $itens = [];

            public function push( {% type %} $element) {
                $this->itens[] = $element;
            }

            public function pop() : {% type %} {
                return array_pop($this->itens);
            }

            // etc...
        }
    %};

    return $ast();
}

$taskList = new_list({% Task::class %});

$taskList->push(new Task('Feed the troll')); // ok

$taskList->push(new NotTask); // oops

// Error: Argument 1 passed to %s::push() must be an instance of Task, NotTask given...