After I merge two branches, the `diff` command shows me that the two branches are still different

I am working on two branches and now I want to merge them.

So I did this:

git checkout MainBranch
git merge SubBranch

That command runs fine.

But now, if I do this:

diff MainBranch SubBranch

it shows me differences in the code between the 2 branches.

Shouldn’t the merge make them the same?

>Solution :

Shouldn’t the merge make them the same?

No. After a true merge, one branch (the one that was merged) is one commit behind the other — because the merge commit is on one branch only (the one that was merged into). If you have this:

x --- y --- z (main)
 \
  a --- b (branch)

Then if you are on main and you merge branch, you have this:

x --- y --- z --- m (main)
 \             /
  a --- b (branch)

A merge commit (m) was added to main, but nothing about branch was changed.

Under some circumstances, Git may do a "fast-forward" when you ask to merge (or pull), in which case the two branches become momentarily identical. But a fast-forward is not a true merge.