Confusing error message: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)

Description

The following code:

<?php

class X {
  const A = '1';
}
$c = 'b'; 
isset(X::{$c}); 

Resulted in this (valid) output:

atal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)

but I cannot use null !== X::{$c}, which also yields a Fatal Error.

With a constant, this message should recommend using 'defined()' instead, or on top of the current recommendation.

PHP Version

Tested on PHP 8.5, but any version since 8.3

Operating System

Not applicable.

首先复现使用动态类常量和 isset() 的 PHP 8.5 示例，然后追踪产生所建议的 null 比较的诊断。确定消息可以在哪里考虑常量，并改为建议使用 defined()；当该诊断对于此示例是准确的，并且现有行为仍得到覆盖时，即表示完成。