Problem/Motivation
Spin-off from #3063887: Support PHPUnit 8 in Drupal 9, drop support for PHPUnit 7.
In order to make the test codebase forward compatible with PHPUnit 8, the big problem is that PHPUnit 8 is changing the signature of the main test methods that test classes extend - above all setUp() and tearDown, adding a void return typehint.
In this issue we explore the option 1 of #3063887: Support PHPUnit 8 in Drupal 9, drop support for PHPUnit 7, i.e. Change the signatures of the methods in Drupal test code according to PHPUnit 8.
While this is not a problem for Drupal 9 core, that supports PHP 7.3+, this will have the impact of forcing testing for contrib/custom modules only on PHP 7.1+ (the minimum that allows specifying a void return typehint to methods).
Setting to critical since per @xjm at #3063887-39: Support PHPUnit 8 in Drupal 9, drop support for PHPUnit 7
If anything I'd want to make the break in 9.0 rather than 9.1 because that's a massive change.
Proposed resolution
Change all the implementations of
setUp()tearDown()setUpBeforeClass()assertPostConditions()
in test classes to have a void return typehint.
Remaining tasks
Discuss.
Patch.