Evil merges in git ?

Question

"man gitglossary" contains this definition of an evil merge:

An evil merge is a merge that introduces changes that do not appear in any parent.

I am not sure I understand the point the authors are trying to get at. Why is it evil ?

Answer 1

Might be because it appears to be merge but is in fact not.

Answer 2

Because it's putting things in the code that no one ever asked to be there. As if you had this code:

$foo = bar;
$baz = qxx;

and this change:

$foo = bar;
$foo++;
$baz = qxx;

got merged with this change:

$foo = bar;
$foo--;
$baz = qxx;

in a fashion that somehow produced:

$foo = bar;
$foo++;
$foo--;
--$baz;
$baz = qxx;

Clearly, this is evil.

I would guess that it's of enough concern to be in man gitglossary because the more involved your merging algorithms are, the more likely it is that they will produce such a thing.

Answer 3

I think it might be named 'evil merge' because it is difficult corner case for "git blame" to solve when annotating file (generating line-wise history annotations).

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Evil merge migh be needed when you developed feature 'A' on main branch, and feature 'B' on side branch, and those features conflict in semantic (non-textual) way. An example would be using the same name for global variable, with different meanings -- this requires renaming the variable for one of features.

For evil merge "git show --cc" has non-empty compact combined diff (but I am not sure if it is equivalence relation; the implication might be in one direction only, i.e. "evil merge" then non-empty "git diff-tree -p --cc").