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

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.