Unexpected or wrong ff, no-ff and ff-only behaviour

Hello,

I'm rather confused about --ff, --no-ff and --ff-only. They seam to be all mutual exclusive while the man page documentation and help reads as if only the first two are mutual exclusive, while the later is a check that can be enabled additionally to the former ones. I'm not sure why I couldn't find anything online about this issue.

The expected behaviour would be that a git merge --ff-only --no-ff only accepts merges where HEAD is up to date and generates then a merge commit. The observed behaviour is that a merge with conflict / non up to data HEAD is entered. If the two flags are swapped it can be observed that no merge commit is created and the branch is fast forwarded.

Sincerely, eyenseo