Merge check to block a merge based on a condition

Writing a custom merge check is quite convoluted because you have to veto the merge request.

It would be more intuitive if we provided a built-in one that blocks the merge if the condition evaluates to true. It could take a custom error message, or a self-diagnosing condition to display a custom message about why the changes could not be merged.

It would be a similar principle to a simple scripted validator/condition in JIRA.