Re: Feature that allows the user to merge only specific files/folders from another branch

"Sugiarto, Titania" <titania.sugiarto@rwth-aachen.de> writes:

What happens when a branch does three different things, and you merge that branch to the mainline but taking only changes for one of these three things? You get changes only to specific files merged, which is what you seem to be talking about here.

You can do so with "git merge --no-commit", followed by selectively reverting the working tree changes to the state recorded in HEAD with either Git tools or your editor, and concluded by "git commit", but anybody doing so must consider the implications for doing so.

We need to remember that a merge commit is this declaration:

   I inspected and understand what this side branch does and what we
   have done in the mainline, and came up with this tree state as
   the merge result.  It is my belief that this result suits the
   objective of the project better than either of these two branch
   tips I am merging together.

After you made such a declaration on the mainline branch, you made everbody to agree that among three things the side branch did, two of them that you did not take were bad idea and should be discarded (anybody who do not subscribe to that idea can go elsewhere and feel free to no longer work on the same mainline).

But what happens 3 months after you make such a merge somebody else (and it could well be you) realize that they want one of the other two remaining changes out of that same branch? As you have already made such a one-third merge, they cannot merge the same branch again to resurrect the two other changes you have already discarded earlier.

You'd need to find these discarded two other changes _somehow_ if you need to resurrect them later, but how?

When a bug is reported on such a state after the merge, the author of that side branch may remember that the bug is addressed by one of these two changes you did not include in the merge, but if anybody asks Git "is the branch that supposed to include the bugfix already merged to the mainline?" they will get "Yes". In other words, they run "git log mainline..three-things-branch" and they will get an empty result because you made that one-third merge. The release manager will scratch their head until they realize what you did, picking only one of the three things the branch did, but recording that as a merge with that branch that did three things.

Chaos.

So,... not really.