Add generic to DeepCopy::copy method

[ruudk]

This makes it possible for PHPStan to understand the return type.

[mnapoli]

Thanks!

[sebastianbergmann]

I think this breaks backward compatibility.

Before this change, the DeepCopy::copy($object) method accepted values of type mixed for $object. After this change, it only accepts objects (according to the new type annotations which are checked by PHPStan, the underlying implementation still handles values of type mixed as it did before).

With this change, I get this when running PHPStan for PHPUnit's code:

 ------ ----------------------------------------------------------------------------------------- 
  Line   Framework/TestCase.php                                                                   
 ------ ----------------------------------------------------------------------------------------- 
  1813   Unable to resolve the template type TObject in call to method DeepCopy\DeepCopy::copy()  
         🪪 argument.templateType                                                                 
         💡 See: https://phpstan.org/blog/solving-phpstan-error-unable-to-resolve-template-type   
 ------ ----------------------------------------------------------------------------------------- 

This is the line of code referenced above: https://github.com/sebastianbergmann/phpunit/blob/5984c2c0052a917262cb8df09c02804eb8e4e5b3/src/Framework/TestCase.php#L1813

[ruudk]

I think we should remove the "of object" on the template. That would make it mixed again.

Would that help? If so, I can make the PR.

[ruudk]

@sebastianbergmann created this as a possible solution: #206