Merge proposal form says "If ... then leave this field blank"

I think I understand the bug, but I definitely don't understand the bug report.

Matthew, could you please attach a mockup? Even a bit of ASCII art would help, I think.

A mundane refactoring would be:

    This branch is dependent on:
    (*) the branch I'm proposing it be merged into (<branch ID updated using JavaScript>)
    ( ) Another branch: [________________________]

However, It's not clear to me why you need to specify the dependent branch at all. Doesn't Launchpad know that already? And even if it doesn't, what relevance does it have to a merge proposal? I'm asking this not out of a wish to see this section of the form gone, but to understand what its labels should be.

Here's the canonical use case:

Jono is working on Feature X (codename "Infinite Jest"), a really big feature that requires significant refactoring of the project codebase. To make life easier for his reviewer, Jono has made a branch called 'feature-x-refactor' that does the refactoring, and another branch called 'feature-x-itself' that adds the feature and depends on feature-x-refactor.

Jono submits a proposal to merge feature-x-refactor and then submits another proposal to merge feature-x-itself, which depends on feature-x-refactor landing first.

In a future release of Launchpad, the merge proposal page will display diffs. When this happens, the feature-x-refactor merge page will show the diff you get when you merge feature-x-refactor into trunk, and the feature-x-itself page will show the diff you get when you first merge *feature-x-refactor* into trunk, commit, then merge feature-x-itself.

This means that reviewers of feature-x-itself can concern themselves with the changes in that branch, rather than the changes in the refactoring branch.

Ok, so the answer to "what relevance does it have" is that Launchpad can calculate the diff. Thanks. But that makes me more curious about my other question: In what situations will Launchpad be able to calculate the diff, but *not* be able to work out for itself that the feature-x-itself branch is dependent on the feature-x-refactor branch?

Launchpad can always guess, but it can never know for sure.

Here's one way to guess. We can check to see if any of the revisions that are in feature-x-refactor but not in trunk are parents of revisions in feature-x-itself. (The clunkiness of that sentence is some measure of the clunkiness of the guesswork.) The problem with this way of guessing is that sometimes multiple branches will fit the bill.

Okay, so something like:

    <source branch name> is:
    (*) A direct branch of <target name synced using JS, defaulting to "the target branch">
    ( ) Dependent on another branch: [________________________]
        Launchpad will calculate the diff based on the dependent branch. <-- formHelp