The Drupal 7 ThemeSettings source plugin does not check that the destination site has a valid theme to migrate settings into

The Needs Review Queue Bot → tested this issue. It either no longer applies to Drupal core, or fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

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Comment #21 added the tag for maintainer review, referring to Comment #18. (I think it means #18.1, not #18.2). Before we respond to that question, it will help if the issue summary is updated to explain what the current patch actually does. From Comment #59:

The scope of this issue is very different from the Issue Summary. The IS needs to be updated.

Also, #30 raised concerns about BC, and #34 has a specific suggestion for addressing them.

I am removing the tag for maintainer response and adding a "Remaining tasks" section to the issue summary.

From Comment #32:

We don't actually have a new plugin id for the destination. So we use the same destination. But because of generated yml files that get exported, one-time, into migrate_plus... we have many cases where incremental migrations will have the updated destination but not the updated yaml that gets generated by the deriver.

Something similar can happen with any incremental migration, whether it uses core migration plugins or migrate_plus configuration. A developer creates some migrations using the core migrations as a starting point, or the derived migrations. These custom migrations include the destination ID. If we change what that destination ID does, then we can break the custom migrations.

Also, a single site migration (not an incremental one) can run into the same problem. A complex project might start today using a target site of Drupal 10.0 (or even 9.5) and the final migration might be into a 10.3 or 11.0 site.

Updated #67 for Drupal 10.2.0.