Feature request

Example code

https://phpstan.org/r/5959b487-8706-43d3-902a-138dd8c00b48

function sayHello(): void
{
	echo 'Hello, ' . ($name ?? 'anonymous') . '!';
}

Description

The code snippet above is valid PHP and phpstan does not complain about it.

However, we can see that the variable $name here will definitely be undefined. Because of that, the expression will always evaluate to the default value. This is a potential source of errors, e.g. when there is a type in the variable name.

It would be awesome if phpstan could detect and report cases like that.