In the process of switching to 0.12 and came across this internal error in some code that worked on 0.11 just fine. I've narrowed the issue down to performing assertions against $this in the constructor which seems to confuse PHPStan.

I have 3 classes:

• abstract base
• abstract frontend extends base
• abstract backend extends base

In base::__construct() I was throwing an exception if $this was not an instance of frontend or backend, to prevent developers directly extending base.

abstract class base
{
	/** @var string */
	private $foo;
	
	public function __construct(string $foo)
	{
                // Remove one of these conditions and you get a different result
		\assert($this instanceof frontend || $this instanceof backend);
		
		$this->foo = $foo;
	}
}
				
class frontend extends base {}
class backend extends base {}

https://phpstan.org/r/fabfb5a1-8b9a-4f13-919e-0a1347869295

It seems to me that after the assertion PHPStan no longer accepts that $this is an instance of base which causes an error when checking if the property access is valid.