First class callable syntax for instance methods

Description

Consider the following code:

class Booking
{
    public function getFreeCancellationDeadline(): ?\DateTimeImmutable
    {
          //  ....
    }
}

Then, you'd like to have a closure:

fn (Booking $b): ?\DateTimeImmutable => $b->getFreeCancellationDeadline();

It would be much neater to have it like this:

Booking::getFreeCancellationDeadline(...);

This is the exact form in which PhpStorm IDE copies the method reference ("Copy Reference" on the method - Ctrl+Alt+Sft+C).

该 issue 未指定源文件、测试或入口点。首先查看提议的一级 callable 语法及其对 PHP 解释器的影响；当演示的实例方法引用语法得到支持并由适当的测试覆盖时，即视为完成。