[PATCH 0 of 1] merge: add --no-fast-forward to suppress fast-forward merge

[Mads Kiilerich]

FUJIWARA Katsunori wrote, On 03/06/2011 11:24 AM:
> The behavior of Mercurial on merge between named branches is changed
> to 'fast-forward' style since 1.8 by 9e7e24052745.

Yes. I proposed a bugfix in 
http://selenic.com/pipermail/mercurial-devel/2010-December/026695.html 
and asked some questions. Matt pushed it for 1.8 without further comments.

The way I tend to see it a merge merges two diverging branches of 
development. Merging with an ancestor is by definition not a merge. The 
data model and most algorithms allows merging with an ancestor, but it 
seems to me like we have an invariant that says that merge changesets 
must merge diverged branches of development.

One place where merging with an ancestor causes trouble is when convert 
cleans up the converted changesets - see Issue2538.

I can however see how the old behavior can be convenient, mostly because 
the current graph visualizations doesn't visualize "changesets on same 
branch are related" very clearly. I think that is the main problem, not 
that we do fast forward merges across named branches. See Issue2678.

A similar partly enforced invariant seems to be "no empty changesets". 
Named branches do however make that possible ... or redefines what empty 
means. It seems like some users/developers would like to see a similar 
exception for named branches and merging with ancestors.

> BTW, I cannot find out any documentation about introducing of
> fast-forward merge in 1.8 release artifacts. Does this confuse
> existing users ?

It is IMHO not something that is introduced. It is a bugfix. That might 
confuse users that rely on the old behavior.

/Mads