Considering merge --dry-run to foresee conflicts ahead of time

I have a half baked alias for this and it proved to be extremely
useful even in this state.

check = "!f() { BRANCH=${1:-HEAD}; BASE=${2:-origin/master}; git
merge-tree $(git merge-base $BRANCH $BASE) $BRANCH $BASE | sed -n
\"/+<<<<<<< .our/,/+>>>>>>> .their/p\"; }; f"

Of course with large conflicts it gets less useful. Getting only file
names from the patch isn't straightforward either.

So my question is what are the downsides to introducing a `merge
--dry-run` option and what would it look like?