3.0 is not compatible with the 2.x branch. This is the whole point of creating 3.0

@nicolas-grekas this should indeed not go into the 2.3 branch (otherwise we cannot release 2.3 anymore until the 3.0 release to be sure about constraints, when we are sure about the compatibility).

So, even if as of now it should be true that 3.0 works with 2.3, that won't be the case for each component once we move forward on 3.0.
What we should do instead is to allow |~3.0 on 2.7, and upgrade all 3.0 components to ~2.7 (and reject anything lower).
That make 2.7 the pivot for upgrading, which is what it is in fact.

@nicolas-grekas even that would not be true either, because we have some place where we will need to break BC between 2.7 and 3.0 (places where we cannot provide a forward-compatible change, for instance the typehint change in form types because of the removal of deprecated interfaces)