[PHP 8] Prepare for the internal functions throwing `TypeError` or `ValueError` exceptions on unexpected types/values

[ayeshrajans]

Description

PHP 8.0 is still in the making, but we know that certain changes are coming up to make things more straightforward and in favor of exceptions instead of rather silent PHP warnings.

One of these changes is that from PHP 8+, internal functions will throw an exception if the function call arguments are of a type that is not expected.

The simplest example would be strlen([]), which would throw TypeError: strlen() expects parameter 1 to be string, array given exception in PHP 8. You can read more at the RFC (​https://wiki.php.net/rfc/consistent_type_errors) and a detailed Change Record (​https://php.watch/versions/8.0/internal-function-exceptions).

WordPress has a PHPUnit annotation with the name @expectedIncorrectUsage that are used to track these kinds of errors. One example is at Tests_DB::test_prepare_sprintf_invalid_args, which uses the @expectedIncorrectUsage annotation to track PHP warnings, but instead, PHP 8's new TypeError triggers an exception (​https://travis-ci.com/github/Ayesh/wordpress-develop/jobs/308369270#L2827).

Proposal

I propose to extend the expectedIncorrectUsage annotation to check the PHP version, and make PHPUnit expect a TypeError or a ValueError. Ideally, we should be expecting exactly a TypeError or a ValueError (sprintf too few arguments for example), which we can pass either from the annotation in the caller, or a check on the parent/sibling classes.