PHPStan configured for php 7.4. accepts php 8.x code

[jacekciach]

Bug report

PHPStan reports "no errors" in the code that would not run in the required version of PHP (7.4).

The problem was detected in 1.9.17. Updating to 1.10.10 does not solve the issue.

Code snippet that reproduces the problem

<?php // php 7.4

function foo(): mixed
{
    return 'xyz';
}

function bar(): never
{
    throw new \Exception();
}


foo();
bar();

Of course, this code won't run in php 7.4:

PHP Fatal error:  Uncaught TypeError: Return value of foo() must be an instance of mixed, string returned

# phpstan.neon
parameters:
    phpVersion: 70400
    level: 9

Expected output

PHPStan should report errors like:

 ------ ------------------------------------------------------- 
  Line   untitled.php                                           
 ------ ------------------------------------------------------- 
  3      Function foo() has invalid return type mixed.           
  5      Function foo() should return mixed but returns string.  
  8      Function bar() has invalid return type never.
 ------ ------------------------------------------------------- 

but reports "no errors".

The problem occurs both in php 7.4 runtime and php 8.1 runtime.

The problem has a quite big impact on the CI/CD - PHPStan didn't report the problem when a developer declared return type mixed in php 7.4 code.

[mergeable]

This bug report is missing a link to reproduction at phpstan.org/try.

It will most likely be closed after manual review.

[ondrejmirtes]

Duplicate of #6861

[github-actions]

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