Bound-Erased Generic Types

This RFC adds generic type syntax to PHP. Classes, interfaces, traits, functions, methods, closures, and arrow functions can declare type parameters; those parameters carry bounds, defaults, and variance markers; type arguments may be supplied at use sites and at call sites via turbofish.

final readonly class Pair<+L, +R> {
    public function __construct(
        public L $left,
        public R $right,
    ) {}
 
    public function swap(): Pair<R, L> {
        return new Pair($this->right, $this->left);
    }
}
 
final readonly class Box<+T> {
    public function __construct(
        public T $value,
    ) {}
 
    public function map<U>(callable $fn): Box<U> {
        return new Box(($fn)($this->value));
    }
 
    public function zip<O>(O $value): Box<Pair<T, O>> {
        return new Box(new Pair($this->value, $value));
    }
}
 
function identity<T>(T $value): T {
    return $value;
}
 
$greeting = new Box::<string>("hello, world");
$paired   = $greeting->zip::<int>(42);
$swapped  = $paired->value->swap();
$result   = identity::<Pair<int, string>>($swapped);